I would try a grain free food. Our 14 year old brittany has eaten a variety of grain free kibble of the years and has never had a weight problem. As we are not the best for long walks I always feed him the lower of the recommended amount. He is currently on Orijen Senior and is working out well for us.
Katharina

Hmmm, he's been on Fromm and Merrick, but they both have oatmeal and brown rice. If it turns out he doesn't have hypothyroid, I will try the Orijen. Thanks!

Weight gain also has to do with daily amount the animal eats, and that includes
treats
and snacks
and raiding garbage cans,
etc.

He does not get an excessive amount of treats or snacks, and he is in the house or fenced yard, so no raiding garbage cans. Considering his breed and age, it is pretty likely he has hypothyroid, but we'll see.

More exercise and fewer (or no) treats should help. If he needs treats, green beans are a good choice, altho' he might not agree. Certainly no leftover human foods. Can you throw the ball for him, get him moving?

Turns out he does have hypothyroid, so he started on meds. Plus, what was thought to be a lipoma turned out to be a mast cell tumor. He is having that removed next Tuesday, we are so hoping they can remove it with clean margins.