
Hi, I'm quite new to the forum too!
I have an Irish Setter bitch, her weight varies quite a lot, if she doesn't eat well for a few days she looses weight quickly because of the amount of exercise she does, she will happily run full pelt for several hours! They are a sensitive breed so any change can upset them, I moved house about 8 weeks ago and it's only over the last couple of weeks that she has really settled in, is happier and eating better (although she has never been a big foodie). She lost weight initially and is now at a much better weight.
Like all have said they are very lean and don't look right (in my opinion) if they carry too much weight. Even though she is a working breed I am putting a coat on her in the colder weather so that I know she doesn't have to use up more of her food energy to help keep warm.
If he still looks lean in a few months keep him on puppy food (Roxy is back on puppy) as this will contain more protein to help hold the weight. At 6 and half months a lot of what he eats will be going towards growing bigger rather than growing outwards, he may have just had a growth spurt!
Jo