Can I ask how old your boy is, also is this the vet you have always been with.
The reason I am asking is weight is often piled on because of life style, and vets it seems always have Hills Science on the shelf [including mine] it is interesting if this person has always been your vet, that he has not advised you previously to get your boys weight down sooner.
My daughters boy has been on Hills Science since he was a pup [now 5 years old and is a picture of health, glossy coat and energetic] he has done really well on it, so I cannot knock it because of what I read on the internet, but he has been monitered for weight increase and food has been adjusted accordingly if he started putting on weight which is not unusual when they get older.
The point of my question is, does your boy need to be on a different food, or should his existing food be reduced and exercise upped to get him into shape.
I realise circumstances can be the reason for weight piling on, either medical problems or a rescue that has not been exercised, and new owner has to try to get them back to a healthy weight, but what I don't feel is right is to change prescription food just to get their weight down.
If it was me unless the dog is getting on in age, I would try to get him swimming and think of a way to reduce his food and calories with some form of exercise, rather than putting pennys in the vets pocket.
I may have got it all wrong but I do not understand why a vet thinks a different food cures all.