This sounds a lot like how DS was when he was younger. He'll be 6 in September and he still has the occasional fit. While it's nice to not have him screaming all day the fits seem to be more intense now. He also didn't sleep through the night or take naps.
I noticed you said that he's on Benedryl. Have you noticed if things get worse when he takes it? It triggers crazy behavior in DS (as does strawberries).
I agree with the previous poster who talked about giving him warnings about when things are going to change. This seems to really help with DS. If we have to go somewhere I start telling him a while before. He can get pretty upset with transitions so he needs this. We stick to a pretty strict routine and even so I still warn him when we're about to move to something new.
I use praise a lot for DS when I catch him doing something good. I don't know about your son but DS doesn't care about any type of punishment. If I tell him that he can't read he'll tell me that he didn't want to read anyway. If I take away his toys he'll play with a stick or a rock. He doesn't care enough about anything for it to make a difference if I take it away or not.
I wish there was an easy answer for this. It does get better but I know that it's so hard. You're already sleep deprived so it makes it even harder to deal with the behavior.
What do you do when he throws a fit? I've tried so many different things. For DS it seems like the fits are just something that he needs to get out and then he's ok. He told me a while back that right before he has them his brain fills up with thoughts and they start going round and round. Then his brain hurts and I guess the fit is him trying to make it all feel better. All I know is that after the fit it's my brain that hurts
What I've been doing lately is holding him really tightly until it's done. There's no trying to reason with him because he's not really aware of what's going on around him. He'll kick and hit if he gets really mad so holding him prevents him from hurting the other kids, me or breaking something in the house. While I'm holding him I try to get him to take deep breaths and I whisper to him to try and calm him down. He said that helps him get over the fit. I think I'm going to teach him to meditate soon to see if that helps too.
I hope that you find something that works for you! I know how completely draining it can be.