Regression: My dog has started toileting in the flat again

No, don't punish him every time he gets on the sofa; otherwise, you’ll end up in a constant battle with him, and it’ll just be a case of who loses patience first. At that game, dogs are world champions! ^^ Give him a firm "no" just as he’s thinking about jumping up, not once he’s already started. You need to nip it in the bud, even if you have to physically block his path to the sofa. If he still isn't listening, then you should start from scratch with his obedience, focusing on rewarding the things he does right. To get him to listen, you also need to do a short training session every day, going over the basics he already knows like "sit", "down", and "stand", while introducing new commands like "on your bed" or "stay". A proper "sit" or "down" shouldn't just last a couple of seconds; he needs to hold it for longer and longer. Keep the training sessions short—10 minutes is plenty—but do them every single day. Above all, don't lose your temper. Make sure to reward him properly when he gets a command right, using play, treats, or plenty of praise. Always end each session with a command he’s mastered, and have a good play session both before and after. Front-clip harnesses that stop a dog from pulling are really effective. Personally, the True Love harness was a lifesaver when I broke my foot and my girl was still pulling. Avoid choke chains, as he might just toughen up against it, especially if he’s already got a strong will and is a bit stubborn. Let’s be honest, putting a few cushions in a dog basket is never going to be as comfy as a sofa or a bed. I actually swapped my dog's bed—which she hardly ever used—for this one instead: https://www.amazon.co.uk/Pawhut-Elevated-Dog-Bed-Navy/dp/B07CPQ8FXL It’s really comfy and plush, and it keeps them off the cold floor. I’m really chuffed with this dog bed; plus, the cover comes off easily, is machine washable, and dries in no time.

I'm going to try rewarding him if he gets down on his own.

It’s definitely attention-seeking, that's for sure. But it’s constant... I’ve tried to distract him from the sofa by throwing a ball, playing with him, cuddles etc... but as soon as I stop, he’s straight back jumping on me or the sofa... The intention is 'cute' but in our day-to-day life, my partner and I can’t even have a hug without him jumping all over us, or if I’m working at my desk, he jumps on me non-stop. And now the real issue is getting more serious: he’s becoming aggressive with us when we’re going to bed or when we leave the flat... He actually bit me this morning! I’m trying to stay consistent: if he misbehaves, he goes into the bathroom for a 15-minute time-out. But now he’s trying to bite me when I lead him by his collar (I do it gently so as not to escalate the aggression).

It’s definitely attention-seeking, that's for sure. But it’s constant... I’ve tried to distract him from the sofa by throwing a ball, playing with him, cuddles etc... but as soon as I stop, he’s straight back jumping on me or the sofa... The intention is 'cute' but in our day-to-day life, my partner and I can’t even have a hug without him jumping all over us, or if I’m working at my desk, he jumps on me non-stop. And now the real issue is getting more serious: he’s becoming aggressive with us when we’re going to bed or when we leave the flat... He actually bit me this morning! I’m trying to stay consistent: if he misbehaves, he goes into the bathroom for a 15-minute time-out. But now he’s trying to bite me when I lead him by his collar (I do it gently so as not to escalate the aggression).

Instead of constantly telling him off, you should teach him how to get off the sofa. This implies that he’s allowed up, but if he understands that jumping down earns him a treat, he’ll do it much more willingly the next time he’s up there ^^ Could jumping on the sofa actually be an invitation to play?

Instead of using a choke chain which hurts him, you should use an Easy Walk harness.

I’m not sure he’ll realise that he’s being shut away just because he jumped on the sofa. If you only punish him every third time he misbehaves, he might understand the very last one if the punishment happens right then and there, but he won’t make the connection with the other times at all. Maybe try buying him a bed that’s much more comfortable than his current basket, especially if he’s been allowed on the bed and the sofa for several months; he’s bound to be quite frustrated now that he’s not allowed on them anymore. It’ll take time. But when you tell him “No!” the moment he tries to get on the sofa, does he not listen to you? And how does he react when you tell him to get down? Does he get down on his own, or do you have to physically move him to get him off?

No more accidents in the house, which is great news! We’ve installed a dog flap so he can go out whenever he likes and it’s working really well. BUT, yes, there’s always a “but”... this dog is so stubborn! He’s started jumping on the sofa again... I’ve had to resort to putting him in time-out (10-15 mins in the bathroom) and then as soon as we let him out, he’s straight back at it... and so on. On Friday night, he kept doing it until I eventually partitioned the room in two. And that wasn't any better; he just barked non-stop in the room... To be honest, I actually had to leave the house for a bit just to clear my head. He’s also started pulling on the lead again, so on the advice of the dog trainer, we’re using a choke chain. It’s a constant battle. We’re staying consistent and keeping calm because there’s no point getting worked up—quite the opposite, in fact—but it is tough going... If anyone has any extra tips, I’d be more than grateful.

I think Mel1 has hit the nail on the head. A happy, and more importantly, well-balanced dog is one that knows its place as a dog! It must feel quite strange for him to realise he can no longer occupy the almost human status he used to have in the household. You need to be consistent with what you’re asking of him (no more getting on the bed, the sofa, etc.). It’ll take a little while, but he should soon understand that the "rules of the game" have changed.