Although the vet says he looks fine, other people have commented on it. He stands about 41cms from floor to shoulder and weighs about 10.5 kilos. If you think he seems underweight what do you suggest I can feed him to help him gain some? He gets about 230g dry food and 400g of wet food plus some extras like about a half cup of goats milk, creamed rice, treats etc. We're in the UK.

Oh my, I went through this with my golden! A real golden, who not only was thin but who never finished his food. He had a dainty quality to him, and he was quiet, everyone called him a Buddha. I got him when he was 10 months old and he had spent the entire winter on a third floor porch, just looking up at the sky, I guess. I even was 'reported' by an insane neighbor who was anorexic herself, LOL (although I have no proof it was her). It was awful. The vet threw the investigator out of his office.
If your vet says he doesn't have worms and is healthy, don't worry.