It depends on a few factors. Is you son over 16? If show he would count an as extra adult and you would pay an extra adult fee. In addition if your are on a package, and you add him before your arrival then he will need the exact same tickets and/or the Dining plan if you have on.
The best way to add he is to add him on the day he arrives by stopping by the front desk to add him. If you do it this way he is not required to have any of the possible additional package offering such as a Dinning Plan or length of stay tickets. Since he is only staying a few days, if he leaves before your vacation is over, just stop by the Front desk and have him removed from your reservation.
He would need his own ticktets but you could make FP+ reservations that include him if you add him to your party in
MDE before the actual trip since he would be part of your Friends and Family plans.
Dave