My 2.5yo has woken a few times after a bad dream with a racing heart rate and shaking too. His language has been quite good for a while now so he's able to tell us he's afraid of (whatever it is). So it could just be a bad dream? My DS doesn't tend to be "wide awake" after a bad dream although I was surprised when I shared a room with him on holiday a couple of months back that he is up and down a lot in the night - all while he's fast asleep

He sits bolt upright and talks and all sorts...
Sorry, bit off topic! Hope your wee boy gets back into an easier sleep pattern soon