I agree that it might be considered a hate crime (attacking a mentally handicapped person is). But this is really a question for a professional. Contact legal aid or even the communications line of your local police department. They can also recommend how to proceed.

I agree too that you need to refrain from becoming the offender. It is best to take the 'higher ground'.

How do other's in the family react to this? The onus is really on them to do something about this and give a stern message to your predator that his actions are inappropriate and will not be tolerated.