I put away the food dishes at night. My vet says it's been proved via video that cats really gorge at night - they're nocturnal hunters, so that makes sense.

Cats can sneak food when we don't even know it!

Also, if he hasn't had a checkup for a while, that might be an idea.

My Oscar is getting quite 'fluffy' and I have him on less food as I want him around for a few years yet.

Good luck.