Kitten meowing in his litter tray

Evening all!

I adopted a Siamese kitten two weeks ago; he's lovely, in great shape, and has already been to the vet. He's eating well (kitten dry food mixed with wet food) and is very playful and curious...

From the very first day, he’s been sleeping in his bed and has used his litter tray perfectly—he’s never had a single accident!! I scoop the tray every day and do a full litter change twice a week!! However, for the last three days, every time he gets into his tray, he starts meowing non-stop while doing his business. I have a few questions:

Is he in pain?

Is he just not comfortable in his tray?

Does he not like the type of litter?

Is there not enough litter in there?

Any advice would be great. Thanks in advance!