My baby is 6.5 m old. he was a great sleeper but since i started feeding him, he started waking up at night crying hysterically.why is this happening?

Sounds like colic: I can't be sure but it's likely colic, a condition where babies have pain from intestinal spasms. We can't always find the cause but i something you are feeding him may be triggering this.If you're giving formula choose organic ones to avoid gmo's. Stop grains- easiest foods are fruits & veggies-try just 1 each day to see how tolerates: http://www.Umm.Edu/altmed/articles/infantile-colic-000094.Htm.

Transition: Between 6 and 9 months babies have an emotional awakening.They realize their primary caregivers are special and become anxious if they are not present.This separation anxiety often alters sleep patterns. A periodic visit at night to let them see you did not vanish is helpful.I would check him from a distance and revisit every 5-10 min until he settles back to sleep. This is a normal stage.