Ok, so I'm with everyone else that I would be totally uncomfortable with this. From a logicistical standpoint, if you can keep this conversation going for a few more days, you will be within the 30 day cancellation period. Since you booked a package, if you cancel, even to shift the reservation, you would have to pay a $200 fee to cancel. In order to book the split stay you would have to cancel what you have because wilderness lodge will be fully booked already I'm sure. You would then have to quickly rebook the room in two chunks. However, there is a not unlikely chance that you will lose the room in the process. While you're grabbing one half someone who is checking regularly for a room could grab the other half. Or, a fun thing that Disney's reservation system has been doing lately is not wanting to book just a couple nights. If you search for 2 nights you may get no availability but if you search for 5 a room all of a sudden pops up. They do this to encourage longer stays. So there is a very real possibility if you release the room you may not be able to rebook it as a split stay.
How do you and your mom feel about splitting the cost in half? I would recommend to her that her and her son can go get their own room somewhere. It is absolutely unreasonable for him to stay with you and not pay his fair share. Not to mention the discomfort and the special accommodations she wants.