[Guest]

No you should be able to do this no problems as long as the room you are booked in can hold your son as well as standard - 4 adults in a room depending on the resort

I'm going through this at the moment with Virgin Holidays trying to get another adult added to our booking for next year, bring on Monday so I can sort it out!

I would call Disney rather than try and do it online, it will probably be easier to amend it over the phone.