Help! Baby not sleeping

I have twin boys who are amazing sleepers at night, well used to be. One of them always woke up once, ate and went back to sleep. He would sleep from 7-6 and it was magical. He's 6 1/2 months old (5 1/2 adjusted) and lately he's been waking up constantly- 5-6times a night! Sometimes eating sometimes I can just rub his back or rock him back sleep. Nothing seems to be wrong and he's a super happy baby except at night.

He has no teeth and it doesn't look like any are coming in so I don't think it's teething.