Hi! My son did this at 4.5 months. You need to implement some sleep training, for your good and for his. We weren't ready to do a "cry it out" method, but we did a modified approach where we don't pick him up again once we put him down, but I go into him if he starts crying. I'll give him a pacifier and go back out. If he starts up again, I put the pacifier back in and might hand him a little stuffed animal or woobie to hold. I try not to make contact and although I might soothe him (hold his hand, rub his tummy, pat his butt or shush him), I don't "play" with him. After a few nights of this, he really started improving. Some books will advocate that you keep waiting a few more minutes before each time you go back in (i.e. let him cry for 2 minutes, then for 4, then for 8), but my son gets too worked up if I wait. What I will do, though, is I will wait to go in if he is only fussing, as opposed to actually crying, but once he is crying I go right in so that I can soothe him more quickly. These things are hard, but they do learn very quickly. My son now goes right to sleep with only one or two very quick "pacifier" visits. He often wakes up about 45 minutes after I put him down, but if I get in there very quickly and give him the pacifier again, he is usually only up for a minute.
You are probably going to get tons of very conflicting advice on this. I'd suggest buying Healthy Sleep Habits, Happy Child, which is a great book with sleep training suggestions and which also tells you the cause of the night wakings. although it advocates a "cry it out" approach, it has lots of suggestions you can follow even if you aren't ready to let him cry it out. FYI, I believe the cry it out method works and in the long run is actually better for the kids, but I myself haven't done it yet. Too much of a wimp I guess!
Good luck!