Nope, he's in perfect health in every other way (without doing blood tests and such). I trust this vet. I work there so I know everyone there well, and I definitely trust them.
Koda is 4.5 years old now. When he was between 1-3 he was 30kg. I think he dropped to around 28 after that. But over the last while he's seemed to keep dropping the weight... so i dunno.
When it comes to following the rule that I know my dog better than the vet does.... I've been thinking he's too skinny for a while now, so the vet's opinion is just conformation of that.