Keep a watch on skin turgidity (firmness) for signs of dehydration. If you lightly pinch the skin (lightly!) and it stands up there's an issue. A healthy snake should also be active if hungry, hunting for food. If listless it is likely in energy-conservation mode and needs to be fed. Or it could be too cold, too, I suppose. But that doesn't sound like an issue here.
It's not really possible to use a ratio of length to weight to get anymore than a _very_ ballpark estimate of appropriate size. Your better off with "if the scales overlap they're too thin, and if they don't touch they're too fat"! Seriously though, the best gauge is this: does your snake seem constantly looking for a way out? If so, feed him. If he just lies there all the time (nights especially, as they do tend to sleep during the days) he's probably hungry and too low energy to waste. If you feed him and after a day or two, he's out and about, he's probably needing more food. I usually feed, wait a few days (2-3 for a snake in your size range), check for defecation, and if the snake is active I feed. If you want to encourage weight gain you could try slitting the feed along the spine with a sharp knife to upon surface area and make the meal easier to digest, freeing up energy for growth. And use the Munson plan guideline as suggested above Raynefyre. It's good advice she gives.....
All this is of course speculative and general. The best thing to do is have him checked over by a vet since there are questions. A physical is good for any new snake and worth the cost of a visit and a parasite check. Depending upon the source of feeders used from wherever you got him the chance is they fed live, and he could have picked up some parasites though pinkies are not major parasite carriers. Or he was in with other young snakes from different breeders and picked something up. Or the store (?) you got him from had contamination issues. Or, or, or. You could also try checking with somebody from a local herp group in your area. It seems like there should be one or 2 in PA. Lot's of members in your general area.... Reptiles USA, the annual issue from Reptiles Mag also lists Reptile/Exotics Vets state by state. A great resource.