You can add him when you check in for your stay just for the nights he will be there. If this brings your room occupancy to more than 2 adults (age 18 and over) you will pay an extra adult nightly fee - approx. $15 to $25 plus tax per night depending on your resort type unless those rates have recently increased.
When you add him at the front desk ask to have him added to Magical Express also so he will have a ride from and back to the airport. ME is not automatic, though it is free (well, included in the room rate) it has to be specifically added.
This adds him to your room, he will not have any tickets - can purchase those separately - and will not be on the Dining Plan either.
If adding him BEFORE your arrival he would have to be included for the same dining plan, park tickets, etc. as the rest of your party. Can only add a person for partial nights when you are on site, but again that just adds him to the room occupancy and does not include any other package features. You can share your dining credits with him if you have extra, or just have him pay OOP at meals.
