DS2, who is 21 months, has been a pretty good sleeper since starting to sleep through at 8 months or so. He self settles, and I put him to bed awake but sleepy. He has always woken now and again, but settled easily after a cuddle, and sometimes settled himself by switching his “Ewan the dream sheep” on by himself.
For the past 2 weeks he has been waking consistently, anytime between 1 and 4am, and getting really distressed, and then staying awake for 3-4 hours, sometimes more.
When I go to him he clings to me, and won’t settle back into his cot, but if I bring him into bed with us he bounds around like it’s morning, chattering away and trying to switch the overhead light on and off! 
I’ve tried leaving him to see if he settles by himself, but he just gets hysterical really quickly. If DH goes to him rather than me, he hits him and screams and shouts for me. He won’t be rocked or cuddled back to sleep, he wants to sit on my knee and cuddle but doesn’t seem remotely sleepy, and as soon as he gets the idea that he’s going back into his cot he gets upset. He’s also started to get upset at bedtime, and often cries for 2-3 mins before he goes to sleep, which he’s never done. Sometimes he eventually falls back asleep after 3-4 hours, but if it’s after 3am he’s usually up for the day. 
Help! I’m getting increasingly exhausted and grumpy, after getting less than 4 hours sleep a night for about 2 weeks. He’s also very grumpy in the daytime! I know “this too shall pass”, but what could be going on? And how do I fix it?!