I think this is something that needs to be addressed ASAP. It is possible your cat is having difficulty urinating. If so, this is a medical emergency!! You should try to get him to a vet, even if it means going to the ER with him.
It is possible he is simply constipated, and may just need an enema, and this is not an emergency. However, since I cannot see or examine your cat or feel it's abdomen, I am going to recommend you be safe than sorry.
If he cannot urinate he will need a catheter passed or his bladder could rupture and he could die. Please try to get him to a vet tonight!
Here is some info on urinary problems in cats:
http://www.marvistavet.com/html/feline_lower_urinary_tract_dis.html
Let me know if you need more help
