Never do business with family, Arew you going to sue him if the horse comes home lame ? Probably not. Always a recipe for disaster and hurt feelings.
Absolutely nothing good can come of this.
The only way I would consider this is to sell him the horse for fair market value. With a buy back first right of refusal for the same amount. If horse is lame or injured or dead, its his problem. You at least have money to shop for another. If horse is perfectly fine at the end of the 3 months, you get your horse back he gets his money back. No horsey changing hands till the money changes hands.