You just have to make his DME reservation...it doesn't matter when he arrives, as long as he is listed on the room reservation. Now...will they give you his room key and park passes?? Can't say for sure. I imagine it depends on whether or not you have him listed for the entire stay. If he is arriving, say, 4 days later, and you don't have him on the room reservation until the day he actually arrives? Probably not. But if he is listed from the time you arrive, then probably.