Do you think he could be having any kind of nightmares/terrors? They can start this young even. If he isnt hurting anywhere, who knows. I only think of that b/c Dylan woke up one night screaming bloody murder and it was after we had watched part of a scary movie while he was falling asleep and even though I didnt think he was watching it, he must have been...it scared us SO bad when he woke up screaming that night the way he did and it took us forever to get him back asleep. My nephew had night terrors when he was around 2-3 yrs old, bad ones...Even if you've done nothing to trigger it like I did, they can occur.
Maybe you should talk to his doctor about this....something must be going on for him to wake up screaming that bad.
Whatever it is, I hope its fixed soon and he starts sleeping better!
Oh and how is he napping?