He knows that he's a separate entity from you and others around him, and may not want to be separated from you at nap and bedtime, he hasn't quite learned or accepted that this is what everyone does. He also has learned he can get what he wants with certain behaviors, so he shrieks and screams until you come in, quiets down when you're there, then starts again when you leave...little ones are so much smarter and able to manipulate us than we give them credit for! Teaching him and allowing him to relearn self-soothing techniques should be your goal at this point, keeping in mind what MandA M said, whatever method you use should align with whatever you're willing to do at bedtime for him for the next couple of years because he'll expect it.
I used a modified CIO with my guy (he's 3.) I hate to hear him cry, but I saw how going to him when he did made him do it all the more :( So, I talked to him before bedtime and explained he needed to go to bed, be quiet and go to sleep, even though he was young and about your son's age (they also understand a lot more than we give them credit for, especially when we're consistent.) I made sure he had his lovey, his bear Arnold, and put him to bed awake, not asleep, same bedtime routine, same time each night, naps as well. Routine is how they learn, and it actually helps them to be more secure as it teaches them what to expect. I closed the door and if he cried or yelled or screamed I waited at least 5 minutes, went back in, didn't turn on the light or pick him up, laid him down, covered him, patted his back, kissed him, said, "Shhhh, time to be quiet and go to sleep, I love you," and left. If he kept it up I waited 10 minutes, went back in, but no longer spoke, just soothed him back to bed, made sure he had Arnold, and left. Next time I waited 15 minutes, and repeated, I never had to do it more than 3 times a night, and he caught on within a few days, but if it continues just add 5 minutes to the time you wait to go in. For the times he does this occasionally (usually after being sick or when extremely tired) I do the same. For naps I make the room quiet, darkened, and never care if he slept as long as he's quiet, and put him to bed earlier if there's no nap. (I'm blessed he still takes them and looks forward to them.)
Bedtimes often need to be tweaked from time to time, depending on schedules, behaviors and such, so since he's getting up earlier i would make sure he's getting plenty of physical exercise during the afternoon so he's tired, and put him to bed a little later, say by 30 minutes, and extending his bedtime routine by 15. Talk to him about his day, and what he'll be doing tomorrow, pick up any toys that are out, just quiet, together time to help him relax. Set the tone now for what you expect, because you will indeed reap what you teach ; )
