I would just continue doing what you're doing now. Decide on a method that works for you and stick with it (sitting in the room, staying out of the room, going in every 5 minutes...) for me, when I had my 12 month old screamer who wasn't on a schedule at home, I let him cry. It took a good 3-4 weeks. Mom and dad were trying at home, but they didn't have the "guts" to let him cry so it took a little longer here than it should have. But now, he's a great sleeper! I think the thing is that you've tried everything, and haven't stuck to one method. If it were me, it would be letting him scream and cry until he passes out and not going in until nap time if over NO MATTER WHAT. I know it sounds harsh, but it works. Luckily, I only had my son when this happened, so I put his dehumidifier in his room on high, right beside his crib and he couldn't hear a sound.

With my little screamer, I would put a toy - usually something that made a bit of noice, it was usually that plastic winnie the pooh book that made noises, and a cup. For the first little while, I even left the TV on (he was secluded in my room) so if he woke up, he had things to entertain him. Slowly, I took things away - he still has a toy sometimes, but no cup or TV. He's not moved into the nap room and shares the room with another little boy. He still thumps a bit in his playpen for about 30 minutes before he goes down for his afternoon nap, but he doesn't keep anyone up and he doesn't cry when he wakes up, or even make a peep. Even now, he's up in the nap room alone (huis roommate only takes afternoon naps) and I'm not even sure that he's sleeping. I'm not sure if he even sleeps the whole 1.5 hours he's in there, but he never makes a peep.

Just choose something, stick to it and BE PATIENT AND CONSISTENT. I would give it about 3 weeks on consitancy before terminating. But that's just me. If you're not willing to put up with the screaming, then maybe you should let him go. But I think that if you just hold off, it will get better. He's so young and maleable right now, it shouldn't take long.

Good luck!