what to do about used truck my boss just bought?

My boss just bought a 1997 f 150 with a 4.6 liter engine in it, and the used car dealer doctored the engine up where it wouldn't rattle and make noise. Two days ago he just found out about it being doctored after taking it to a mech shop because the engine just started hesitating and losing power. The mech was a certified mech and showed him the sludge that came out of the engine and told him he had to refill it with oil, run the engine and drain it again. He told my boss the engine was doctored on purpose, and the engine was shot and would cost $2000 to replace. Well my boss went and told the used car dealer he bought it off of and played dumb an tried to trade him the truck for another truck. The dealer went to naming off things that were wrong with the truck he wanted to trade the 150 on. Right then my boss knew that the dealer didn't want his truck back. He then told the dealer he knew the truck had ben doctored and that he should fix it, and the dealer said no, he lost money when he sold it to him for $4000. My boss is a great guy but he don't know what to do, what would yall do.