You may want to take him to the doctor to make sure nothing physically is wrong (ear infection, etc.) - and then the doctor will probably tell you it's "sleep resistance". The doc may give you some info on it...there's also some info out on the web. We just went through this a few months ago with our two year old.
Apparently, it's a totally normal two-year-old thing and is a phase. The important thing is to address it head-on and be loving but consistent.
Here's what we did: Our little girl has a "big red pumpkin" (the kind that holds candy for halloween) that she gets to pick something out of after dinner. We started saying, "if you go to sleep with no crying and no fuss, then you can have big red pumpkin tomorrow". We gave her time to settle down if she started crying (I never went past 7 minutes), and then if we had to, we'd open the door (not go in the room) and say "You are okay. It's time to sleep. I love you. Night night." and then close the door. At first it took a few times of this. It got much better quickly once she figured out that we weren't giving in (this is where the consistency comes in). Throw a stuffed animal out of the crib? I'll get it for you after you go to sleep. Want to be covered up again? I'll cover you after you go to sleep. The next morning first thing, we'd either talk about how sorry I was that she "lost her pumpkin" because she cried before she went to sleep (but that we would practice again tonight) or we'd celebrate (LOTS of praise!) that we get pumpkin today because she went to sleep like a big girl with no crying and no fuss.
We practiced this for naps and bedtime and we talked about it at other times of the day to (cheering her on if she did well the night before; concentrating on what needed to happen (not dwelling on the night before if it went badly) that night and how she'd get pumpkin the next day if she was a big girl.
A long response, but I just went through it, so I feel your pain. It DOES get better. Just remember: love and consistency.
Good luck!
K.