Are you visiting Orlando during your trip?, Mickeys not so scary Halloween party is great fun and also involves trick or treating! They run on several nights through Sept / Oct so you wouldn't need to be there on 31st to take part.
If he's wanting to do trick or treating on Halloween I'd phone or email the Ft Lauderdale City Hall and ask them what's going on in the community. I guarantee there will be stuff to do- trick or treating, Halloween games etc. Some towns now have organised trick or treating times in certain neighbourhoods and the city hall should be able to point you in the right direction.