L.D.
I wonder if he is going through a growth spurt. My kids always seemed to be more alert and more active for a couple of days leading up to a growth spurt. Also, since you are feeding him rice cereal now and possibly formula (not sure if you are supplementing with formula right now), consider the possibility of acid reflux or some kind of food intollerance. If that is a problem for him, that could cause some discomfort when he is lying down in bed.