In order to use Magical Express your name must be on the room reservation. So unless you have him officially added to your reservation he can not use ME. And everyone booked in the room must be on the same package with dining. I am not sure how that would work if he was arriving a few days later though-- you may have to just go ahead and pay for the days that he isn't there and then you could take advantage of the extra credits to eat at a Signature Dining location.
So you have advantages and disadvantages to following the rules. With him registered he can use the transportation. But there is also an additional charge for each adult after 2 in the room. By not following the rules and leaving him off the reservation you won't have to add the dining plan for his use if he doesn't want it, but you can't add it if he does.
If you are planning to take advantage of any of the Extra Magic Hours either before or after the park closes he won't be able to participate if he is NOT on the room reservation because he won't have a room key.
Additionally if it is found out that you have an unregistered guest in your room Disney can ask you all to leave (or more likely just pony up the extra money!). If you also have children with you (sorry, didn't notice if you said you did) and already have 4 in the room then adding him as a 5th (unregistered) would be illegal and would result in greater consequences if discovered since just paying the extra adult supplement is not an option.
