Hello there,
Our puppy was (and still is) a notoriously 'anorexic' pup (she's now 11 months old). To give you some idea at one point she went for nearly 4 days with no food (but was otherwise herself)

... and if she's had enough she'll even leave food with Nature's Diet/tripe mixed in with it

. Most people (including her breeder and our vet) comment that she was/is a little on the lean side - but that this is miles better than a little fat :). It helped when people said I shouldn't judge her to other dogs around here as a lot of them were probably a little overweight. We kept reminding ourselves that dogs won't knowingly starve themselves and stuck to the 20 minute rule and no treats inbetween - which seemed (eventually) to kind-of do the trick. Mackerel in tomato sauce added also helped, as did a raw egg (shell and all).
One thing that helped us was to ignore her at mealtimes - the more we 'encouraged' her the more finicky she became. We've now got her on Arden Grange Prestige which has filled her out a bit - but I don't know if this is suitable for young pups.
For training we use baked liver chopped up. It smells rank when cooking (I'm vegetarian so you can imagine how much I like it

), but it seems to do the trick. The only trouble is that every other dog in class won't leave me alone

. Another thing that worked was getting her toy focussed since she prefers play to food.
The next pup I have I want a total dustbin - life would be much easier :) .. hope that helps ...