14 month old waking at 3am every night for 3 or 4 hours??!!!

[999HELPMYPUDDINGSONFIRE]

I have him along with a 9 week old.
He's always been a bad sleeper until about 3 months ago where he started sleeping through.
He's never gone to bed before 11pm but started sleeping until 7am which was fine.
The last 10 nights he has woken between 2 and 4am and is staying awake until about 7am?!?
He doesn't want anything to eat or drink and even on the rare occasion he does accept the warm milk he still doesn't go back to bed!!
Any advice?

Cover off the obvious things like too cold, too hot, too much noise (new baby disturbing him?), too much light first. If he's crying when he wakes up, teething could be a reason. If you don't think that's the case and/or if a shot of painkiller doesn't help, then try a gentle 'shh... go back to sleep' and leave him alone for a while. If he's not crying then leave him alone and he may end up settling himself back off through boredom. Good luck. Broken nights are horrible.