Considering that the Sproutling has had less than six weeks of life experience, we must assume that unless he's drawing on some previous lives (and who would care to speculate on those?), his dreams at the moment must be revolving around boobs, cuddles, cold wet feelings on the bum and a plethora of ever changing faces.
Just after he falls asleep, Sprout goes into a dreaming mode where he can wriggle around a fair bit, roll his eyes while they are closed and coo. Sometimes he smiles and even giggles in his sleep, something he has done a few times while awake but not regularly yet.
Dreams are very important for our brains to process the things that have happened during the waking hours. Babies spend more time in REM (dreaming) sleep than adults do and it's thought to be an important part of their brain development to try to make sense of a new and amazing world.
I'd love to know what it is he's dreaming of, but all I can do is guess. It's certainly cute to watch though!