Just make sure you take it very slow. If he used to be in some pretty intensive training, then he may have old strains, scar tissue, etc. that you don't know about. Also, an older horse's joints, ligaments, muscles, and other structures won't heal as fast after exercise as his younger counterparts. Try some long walks at first, then progress to trotting, hillwork, cantering, and then jumping. Always evaluate your progress during the process. If he feels stiff or lethargic on any day, then give him the day off, never push him too hard.
Good luck with the lease :)