28 month old crying all the time
It sounds like he's talking well, have you tried to ask him if something hurts? My 2 year old is going through intense tantrums but when they're over, she's fine. It helps if I sit on the floor nearby and say "mommy is here to give you hugs when you're ready" and she usually comes over and settles after a while. It worries me that your DS doesn't seem to be content for stretches of time. I'm sorry about the questions but I'm trying to get a better picture.
The days where he gets only 8 hours of sleep don't seem like enough. I think it's a good idea to try to reestablish a nap. If that doesn't work then a car or stroller ride where he has to be still for an extended time may help him recharge - we do this on rare days when DD doesn't nap. Failing that I'd try quiet time: where the two of you snuggle up on the bed/couch/rocking chair and read or sing quietly. The downtime should help reenergize him a bit.
Growth spurt (including development such as talking)? When my son goes through growth spurts he is like velcro to me , plus he eats alot and sleeps alot and is very irritable.. Lasts a few days but not more than a week. Then I notice either he has made leaps and bounds in speaking, or some other skill. Other times I note that his feet are bigger or his pants shorter.
Does any of this seem like it could be the case for your little one?
Apart from that, as mentioned by PP.....the possibility of ear ache from the cold that he had.
Also as you said, sleep. He may need more. Some kids may get by on what he does but many seem to need more.