Retroactively turning down an accepted offer to a professional program from one school in favour of an offer from another school?
The first school won't like it, but there would be nothing to stop your brother from backing out of the first offer and accepting the second offer, unless the schools work closely together to prevent this from occurring, which seems highly unlikely, especially because their notification dates differ so greatly.

If your brother is accepted to the second program, the most courteous way to inform the first program would be:


as quickly as possible, so they can accept someone from their waiting list or otherwise make use of the information.
without insulting the first program.


For example, he might write something like:


  I realize that, as grateful as I am for admission into your program, pursuing it would not be the right thing for me to do at this time and that I should not have accepted your offer.  I am very sorry that I have to withdraw my acceptance.  I hope it is not too late for you to offer the place to a student who is a better fit for your program.