This is awful, it’s not normal at all. An open wound where the edges aren’t meeting just won’t heal properly, and it lets all the muck in, which leads to infection. The vet needs to stitch it up and leave the sutures in so the edges can pull together and the skin can knit back and heal. If needed, you should put sterile gauze pads on it (you can get these in the first aid aisle of any supermarket) and then a bandage all the way around the thigh (again, you can find these in the pharmacy section or just order a few rolls from Amazon as you’ll get through them quickly).
You'll need a large buster collar for the dog (the vet usually has some they can lend you).
As for the dog, you’ve got to avoid all exercise—no stairs, no running, no chasing balls, and be really careful with 'dirty' areas like sand, mud, water, etc. It’s tough, but it’s for his own good; otherwise, every time he moves it'll pull on the stitches and the wound will open back up.
My dog once split himself open from his paw pad right up to his dewclaw (about 3 inches). It wasn't deep, so they just used staples, but it was the same deal: no long walks, no playing, changing the gauze and bandages every 2 or 3 days, plus some healing cream. It took three weeks to close up properly (it probably would've only taken two, but on the fourth day, the little monkey managed to pull his staples out...).
Mind you, my vet offered a bit of a package deal: I paid for everything upfront the first time (the consultation, the staples, gauze, and bandages) and that included two follow-up visits (one in the middle to check on it and one at the end) for about £50, which covered any extra staples or cream needed during those appointments. If we went over those two visits, I’d have been back to paying the full rate again...
