Crystals in his urine are definitely a possibility. They are sharp and can cause bleeding.... (my poor boy had some blood when he had his bout with FUS). Take him to the vet ASAP - it doesn't sound like he's blocked yet, but it will be bad if he does become blocked (very painful for him and $$$ for you). They'll get some urine and test it, then probably put him on a crystal busting food - it makes very acidic urine to break the crystals down.