So maybe the teething is particularily hard on him. As for the pooping - nothing going in, nothing coming out. If he isn't eating enough for him to process through his system then there would not be much coming out the other end.
Have you tried different foods with him? Maybe he is tired of what you are feeding him. Petsmart isn't the greatest store for a good variety of foods. At least not in this area they aren't. I find much more success at Pet Valu or Global. Global with order foods in for you. Pet Valu probably won't but they have a good variety of foods. Try to stay with the grain free, by product free foods if you can. As sugarcatmom stated earlier in the thread you don't have to feed him kitten food. Any good quality food will work at this age.
I found some of my kitties at 4/5 months got a little pickier with their foods too.