How about:
-Offer to sell him the house/lease option it to him, possible at a favorable price (to you, since he seems to love this house)
-Get something in writing that allows him to continue his "hobby", but allows you to approve work actions AND clearly states you will not be reimbursing him or paying him in any circumstance for improves he does decide to implement on his own. You can always make specific instances of payment for specific work products, again, each one should be in writing. And make him pull permits where required, or at least use that to throttle work. "I checked, that requires a permit and inspections, if you really want to do it, you'll need to follow that process...." For example, my city actually requires a permit for a shed (and a bleeping concrete pad...)
Personally, I would try to work out the last option, he's a good tenant, is improving the property, not trashing it, and if you can get both control of work scope approvals, and free improvements, it could be a win-win.