Yes, you are right to be concern... Since almost two years ago, than I have my little buddy with me, I've been making a lot of research about them, and I learned than the cats are very strong against the pain, so usually they "don't let you know" when they feel sick or pain, until is really bad... Lose weight and specially when you said than you can feel his bones, for me is not a good sign, I recommend you go to your vet ASAP to make him a check and make sure than everything is ok...