given the picture and the fact he was on the road and his parents were around hes fledging...meaning hes learning to be a big bird...
at fledging birds will exit the nest and typically spend a week or so on the ground learning to eat and practicing flapping the wings untill eventually those wings work well enough to get them airborn and thats it...
the parents will continue to feed while on the ground at this point but baby woud also be responsible for finding his own meals too.
personally i would do ground dog food mixed with some water for your feeds, then put a couple of small worms (or cut the worms up) in a jar cap for him to pick at you might also want to sprinkle a little mixed seed (millet is good) on the bottom of his box to start picking at.
i would put a small STURDY dish of water (keep it small babies often drown in waterdishes) or will tip it.. for him to start to learn what water is too...
but dont be suprised if hes ready to go pretty soon.
hopefully the rehabber will get back to you because unforotunatly it is illegal to keep him even untill fledging...
but if not he will fairly quickly finnihs feathering out and start to try those wings...once he does itll be up to him as to when hes ready to "leave the nest"
good luck