Keeping your boat registered in FL would not necessarily guarantee keeping your FLorida residency, (seems like they are always looking for people who try to do that to avoid taxes). But keeping everything else the same (voter''s reg., driver''s license) may. Why don''t you obtain a P.O. Box, a cruising address, at a place near where he lives instead of his home address? Maybe not at a U.S. Post Office, but at one of those mailbox rental places? People cruise all over all the time using these and still keep their residency in their home states.
And, have fun,