Any way you can get him to a pool three times a week for 10 minutes at a time? One minute of swimming is equal to 4 minutes of running.
Has he had a senior workup done by the vet, lab work? I would rule out medical causes for the lethargy/weight gain before trying to find the perfect food. Don’t assume his symptoms are age related.
See what the vet recommends.
Nutrisca Salmon and Chickpea worked well for my senior, see chewy dot com for reviews.
What does your dog weigh? What does the vet recommend he weigh? Weigh him once a week by standing on the scale while holding him. Weigh yourself first, then deduct your weight.
If you have pet insurance and the vet prescribes aqua therapy (swimming in a heated pool) for medical reasons/rehab, weight loss….it may be covered.
PS: Add water to the food and take him out to void (urinate) often.
