Sorry, but your vet sounds like a total ….. Mhhmmm. LOL.
I would start by getting a rabbit-savvy vet to check over your guy because this vet obviously has no idea what he’s talking about. I’m assuming the two bunnies were bonded and the loss of a mate can be extremely hard on a bun, especially if not provided with the proper time to mourn and to acknowledge what has happened (for example, leaving the dead bun in with the other one so he can understand the situation). I’m assuming this didn’t happen as you said you were forced to put him to sleep which is understandable and not many people actually know they should/can do this for the other bunny mate.
As Thumper said, you should definitely try enticing him with his favorite foods so you know he’s at least eating something. What is his normal daily diet and how has it changed since you lost the other bun? I would start by giving him some of his favorite veggies. And spending A LOT of time with him. I imagine your bun is feeling lonely and depressed (just like a human would) so any comfort you can give your bun will surely help them begin to heal.
I REALLY urge you to find a different vet to take your bun to though. Loss of weight can be a symptom of so many things and a rabbit-savvy vet wouldn’t just write that off as nothing. Do you know if he checked his teeth? Teeth problems are a very typical cause of buns not eating as much as it pains them to eat. I would get this ruled out right away. If it IS physical and not emotional, you’ll be able to save yourself and your little bun a lot of stress and possibly pain.