If his treats are veggies and other low-cal stuff, maybe you should look at howmuch he eats at meals and reduce that a little if he is gaining too much weight? If you give treats such as bonios/markies you're meant to adjust the quantity of food they have at a meal - this is easier to do if you're feeding a complete food.
I give treats for being a good girl basically, but they tend to be tiny ones, half a mini bonio, or a dog choc drop - I'm a firm believer that the thing your dog most wants from you is lots of praise and lots of petting.

Lynsey and Tilly