My only other suggestion is to have the full or expanded Thyroid profile done, the thyroid results that come back from regular blood workwork just are not definitive enough to rule out any thyroid problems. A good evaluatory lab is
Home.
Also do not feed evening meal late and walk him after meal - or have a play session etc.
Underwater treadmill work is great, or even a ppl treadmill is good stuff. Come December we have an indoor doggy swimming pool I will take the pups to at least once per week.
When I need to put on weight on my males - I feed a good portion 2/3 of the daily allotment of food in the evening after their last walk. I also will bump up Razzles' food allotment by 1/4 c. Usually I need to do this all summer as the boys are most active then.
Another trick is to measure their treats =ie in 1/8 cup size container - that way you do know how much extra they are getting.