Hi Erin! Welcome to the Disney Parks Moms Panel!
The MagicBands are so much fun. I can understand why your son would have such a hard time choosing. I have quite the collection going myself!
I found some information on the Disney website that seems to indicate that once MagicBands are assigned to a specific person in your My Disney Experience account, they can't be changed. In addition, if your son is under 10 years old, he will have been assigned a child's ticket. My little guy and I accidentally switched MagicBands once, and although he was able to enter with my band, I got the dreaded blue light when I put my finger on the scanner. Apparently, if the system is expecting a child and sees an adult, it won't allow entry (fortunately, a helpful Cast Member was able to correct our mistake).
If you want to chance it, you could try going to "Magic Bands and Cards" in your My Disney Experience account and see if you can switch them there. Since they haven't been used yet, you may be able to reassign them. Again, I'm not sure if it will work, but you can take a look and see what it'll allow you to do. Another option is to call Guest Relations and see if they have any advice for you.
I hope this helps with your planning, Erin. Please come see us again if you have any more questions about your upcoming vacation. We love to help!
Keep Disney Dreaming,
Kathy K.