J.L.
Hi J.,
As a pediatrician we see and hear of all sorts of sleep related issues. Night terrors may start as early as 18 months but do not start at 6 months. I was sad to see one of the responses suggested that it was due to immunizations. Yes he may be a bit sore and irritable after his shots and this may set him up for a sleep disturbance but protection from these diseases is so important. More likely he may be having teething or gas discomfort or may simply be over tired.
I would try to stick to a good routine. In terms of what to do when he cries at night- comfort him. With that said, this is the time to develop good sleep habits. If you can comfort him but leave the room before he falls asleep this is great. You want him to be able to fall asleep without your help ie rocking, feeding etc. This is all easy to say and hard to accomplish. Just know that things tend to get harder as they develop object permanence and seperation anxiety between 7 and 9 months of age. Hope that helps and hang in there.
J.