Hi Melissa,
The final call about whether a costume is appropriate for Mickey's Not So Scary Halloween party is up to the cast members on site. This means I can offer advice, but I can't make any promises about your son's costume.
The general rule of thumb is that Disney does not want guests to wear costumes such that the guest might be mistaken for an actual cast member. So whether your son's costume will work depends on how realistic it is. One year I attended MNSSHP with former
Moms Panelist Bret. He dressed up as a Jungle Cruise attraction host. He wore an all-khaki outfit very similar to the actual Jungle Cruise cast member costumes. However, instead of a standard size cast member name tag, his faux name tag was about eight inches across. This difference made if quite clear that he was a guest at the Party and not an actual cast member. If your son dresses exactly like a cast member, he will likely be asked to change. If he dresses as a parody of a cast member, he will probably be OK.
While MNSSHP officially starts at 7:00pm, the holiday party mood often starts as early as 4:00pm. Again, the cast on site will make the final call, but in my experience, if you're in costume after 4:00pm on a Party day, then you'll nearly always be fine.
Please let us know if you have additional questions. We're happy to help.