The pilot, father of the passenger, a manic depressant, said that his 29 year old son seized the controls while cruising at 5500 MSL, and attempted to force the aircraft down into a lake. The pilot was able to overpower his son and recovered at about 3500 ft MSL. Then, his son shut the engine off with the ignition keys and threw the keys out the window. Once again, the son attempted to take over the controls and succeeded in putting the aircraft into a spin. The pilot was able to recover from the spin about 50 feet AGL, but the aircraft was nearing a stall and he was able to guide the aircraft between two tall trees. The aircraft cut one tree off and uprooted the other. This resulted in the aircraft losing most of its forward speed and it fell to the ground from about 30 feet AGL. The pilot and passenger suffered minor injuries and the aircraft was substantially damaged. The passenger was placed in a psychiatric unit after the accident.