Not sure on what rules specifically apply, but normally you do not practice in pads for the first three or four days. I'd almost go with that idea in this case.
As for when they can play, it's a coaches' decision. I certainly would expect that a player would have practiced with his team in pads for the week before the game, but if the kid came from another school/program (transfer) where he played football, I might be leaning towards a quicker entry.
In any case, do what is safe and right for the kid. Injuries can be severe and I'd rather not have that reoccuring nightmare where I'm waking up in the middle of the night, replaying something over and over and wondering what would have happened if I did "the right thing."