What is the most conservative route that I can propose
to him, for Mother's sake? :-)

The most "conservative route" is to cross over to Cuba. Check in,
they will be happy to see you and not stamp your passport. Then go
down the coast to Havana, there are several places to pull in along
the way if the weather is turning poor. Once in Havana, wait for a
weather window and then cross over from Havana to Key West. It is 90
miles from dock to dock. Once in the keys they can go up the inter-
coastal water way to Miami. Somewhere in the Keys they will need to
check in with US imigration/customs.

-Mark
http://goreads.com