Kitten still sneezing but doesn't seem to have a URI

My kitten who is 3 1/2 months now (adopted him) has been sneezing off and on since I got him a month ago but he continues to be healthy in every other way. I would hate to have to go to the vet and for them to charge me their inflated prices just to look at him and say he is ok or request that he have other tests done. Could it be that this kitten has allergies and what is the best thing to give him for that? I have heard you can give a cat liquid Benadryl. He is about 4.5 lbs. now. How long can URI's last?

It's very possible he has allergies.  Unless he's sneezing with colored discharge, I wouldn't worry about giving any antihistamine.  Also, look for weepy, watery eyes.  If the discharge turns color, then to the vet you go!  You can add some Lysine to his wet food (250mg a day for his age) and see if that helps.