What is his age and activity level? Maybe his metabolism is just slowing down and you need to adjust his caloric intake accordingly.

I would get some bloodwork done if you have a yearly or 6 mos. checkup in the near future....just to rule out bigger problems.

As others have suggested you should probably measure his food carefully and enforce a meal schedule. I would also try to increase his activity if you can find some toys or games that interest him!

Good luck!