actually no, not looking for people to make me feel better. I don't think that 5 days and 6 years are comparable. I also don't think his plan is to not pay for a park ticket. He just doesn't want to pay more than double for a room. The question was more of if proof was necessary or if he would be forced to upgrade rooms when he got there.
Two issues here...
One, if the child is going to have a ticket, it must be linked in a
MDE account. In order for him to have the 60-day FP+ reservations (which everyone else in the family will probably want), he must be listed on a reservation. In order for him to board the DME bus, he must be listed on a reservation too.
That leads to issue #2...the system will not allow five people to be listed on a reservation for a studio at SSR. This means that in order for him to be on the reservation, he would have to be listed as under 3, which means no ticket can be linked to him.
Your brother will either need to lie about his age and skip buying a ticket and stay in a studio, or be honest about his age, buy the ticket and book a room that accommodates 5.
In regards to what I bolded above, yes, he might be forced to upgrade
subject to availability. The points would be taken from YOUR DVC account (he could not pay cash on the spot) and if there is nothing available, he may be turned away altogether.