Congratulations on your new family member!
I went through the same ordeal last year with a very elderly and underweight rescue; how to put on weight safely, finding the balance between feeding him enough but not too much, so that his weight gain wasn’t too quick and his system wasn’t overwhelmed with too many fats.
I second the advice above – if at all possible, feed him 3 or 4 small meals a day instead of 1 or 2 big ones. I fed a good-quality puppy food mixed with a good-quality adult food* starting with a 3:1 ratio. As he came close to his target weight, I gradually changed the ratio to 2:2, then 1:3, and then finally all adult food.
Supplements: don’t over-do it. I’d give him probiotics and maybe a joint med, but really, good food is going to wonders all by itself.
*Earthborn Holistics Puppy Vantage, Earthborn Holistics Great Plains Feast, Earthborn Holistics Large Breed (all dry); Wellness Complete/Core Puppy, Wellness Complete Senior and Canidae Large Breed (all canned). I mix up what I feed my dogs; variety is a good thing.
