*personally* i think they can have nightmares... and with that kind of crying, i'd definitely go in and comfort him & send him back off to sleep.
however, might he just be calling for the bottle? dunno if that's something he's used to, or if you just started it now because of the sudden screaming...
could also be early separation anxiety. i know luke has a horrible scream when he's in the middle of that....
okay, i'll stop rambling now