I've heard from people who have had this problem that these are the solutions that have worked:
-- Get in the bath with him
-- Get in the bath and let him watch you from outside the bath - splash around, have lots of fun, invite him to join you, but don't push it
-- New bath toys - boats, squirters, bath crayons, things like that
-- Let him play in the empty bathtub -- it's a great place for messy projects - paints, markers, etc
HTH!