How many sets of ribs are you seeing? You
should be able to clearly see the last couple of sets, and on many pups you can see more. This is NOT unhealthy. Overweight is unhealthy, and far FAR more likely to do damage to a pup than being a little underweight.
If you can clearly see all ribs, then he's a little underweight (but certainly not problematically so). If you've got hipbones and vertebrae prominant as well, then it's a bit more of a concern. I'd be cautious about trying too hard to fatten him up though. In many cases, this is just how some pups grow - they put all their energy into growing upward as quickly as possible and simply
will not fill out until all that upward growth is done. Feeding more just accelerates the upward growth, and that can be damaging.
If you really are at the point of prominant hip bones and vertabra though, then adding healthy protein into his diet in the form of fresh lean meat, fish, the occassional egg or canned fish (actually, these are all good healthy things for her to have in her diet anyway

). I'd just be wary of increasing the overall amount of food by a great margin over guidelines. Overfeeding won't do her any favours, whilst being lean will.