Inconsolable at 5am

My 5 month old boy, who was the easiest baby until the 4 month sleep regression hit timed exactly to him growing his first teeth, is really putting me through the wringer at night time. It takes about an hour of rocking and singing to get him down (start around 7, he’s finally asleep by 8) and then from them on in he’s waking up every 20-40 minutes all night. I’m the only one that can calm him so my husband can’t even take shifts. When he wakes it takes about 10-15 minutes of walking/bouncing and singing the wheels on the bus (I now hate that song) before he’ll go back down for about 20-40 mins each time. Then, 5am hits and he is inconsolable. Crying so hard, and when I do eventually calm him he will only sleep in my arms. I can keep that going until about 6am, then it’s game over and up.

Any suggestions for why he’s SO upset at 5am? I have tried an earlier bedtime but I also have a 4-year-old who demands mummy does bedtime so it’s a serious juggle!