L.B.
I think it's because he fell asleep next to you and when he wakes up (all babies wake up periodically through the night), he's old enough to realize that you're not there and he doesn't like it. Unfortunately, sleep training may have to be done. Both my kids had to cry themselves to sleep. Even if I was just holding them, they often cried. My doc said it was just a release of their energy. Even my three year old sometimes does it when he's really tired. Definitely read Ferber, it will help you understand why he's waking up and crying. He helped me understand the physiology of sleep so I could let my babies cry and understand that they needed to learn how to self soothe.