FAA part 91, section 119 - Flying over congested areas of a city, town or settlement is not lower than 1000' and over other than congested areas not less than 500' and over sparsely populated and water you may go lower but not within 500' of a vessel or person. This does not include during takeoff and landing.
I saw him about 1:30 while I was on Havana Golf Course, he was skywriting. Looked like he was trying to make a shark, but the wind was too strong for it to last. It looked like a Pitts bi-plane. I don't believe he was less than 500'. If you had his tail number you could report him to the FAA, they would probably ask him if he was less than 500'.
FAR Part § 91.119: Minimum safe altitudes: General -- FAA FARS, 14 CFR
