Congrats on the upgrade!
Unfortunately CRO/WDWTC is quite strict on occupancy - and as already posted, the official max for a 1 bedroom is 4 persons plus an infant under the age of 3.
That said, DVC members are currently allowed 5 persons in a 1 bedroom even if all of them are over the age of 3. But no extra bedding beyond what is already in the room is provided for the 5th person. Many members bring an aero bed or similar for that extra person - DVC resorts do not have rollaways or cots.
If it were me, I'd call CRO/WDWTC and see if they will modify your reservation for 5. (Doesn't really make sense to have different rules for cash guests vs DVC members, but I think they do). If CRO/WDWTC says no, then you really can't invite more than one friend for your son. Unregistered guests don't get room keys and thus can't go to EMH or use ME. It's also a problem if you plan to add the dining plan to your reservation.
Good luck - hope it works out for your son to invite his best friends.
I'm closing the thread - anyone who wants to discuss occupancy can do so in
this thread.
Tink-aholic - Please don't feel that you have done something wrong by asking the question. We limit discussions of occupancy because many posters here have very strong opinions about the subject and sometimes they forget their manners when expressing those opinions.