@alanjolly65
Sorry if I'm being a Debbie Downer about your Cunning Plan. If he's paying for everything, I suppose there shouldn't be much downside. However, to me, it sounds like an ongoing aggravation, and personally I'd avoid such an arrangement, especially when there are viable alternatives.
I also question to what extent you're "not paying for it". The immediate issue is what vehicle is it, and who'll pay for it. Even if he doesn't have expensive tastes, I'd imagine 10k-20k euros is easily spent. There are only two options: you buy it as it's in your name (which means now you've spent 20 grand, and you might lose it if he writes-it off in the first week of his annual holidays). Or, he gives you 20 grand... and now he's driving around in a vehicle that HE paid for, but it's in your name, so you can sell it / rescind the informal use agreement at any time, and keep his dosh. The latter is better for you, but both options are bad for one of you. Or, maybe, he picks out a car, agrees how the arrangement will work, and sends you off to buy and register the vehicle. Having done all that, he comes over... finds some little issue that he doesn't like, and decides he's not so keen on the car/arrangement after all, and decides not to reimburse you. And leaves you out of pocket for a car you don't want/need. But maybe I am I way too cynical in my old age! :-)
Given that he's retired, I'm not sure there's much to be gained by NOT becoming a resident. With a pension it's pretty easy and fairly inexpensive. He can still stay 5 or 6 months (or less), but he'd have the extra flexibility of staying longer, or exceeding the 3 months limit... and without any nagging concern that he might accidentally overstay. It also would allow him to be in the local health system (NHIF) or buy his own car, should either of these ever prove to be useful.
I'm in a similar position regarding Cyprus and Spain. I'm very comfortable in Bulgaria, and I have to intention to "move" to either of those countries. But I find it convenient to be a legal resident there, rather than a tourist, and I can enjoy my property whenever I feel like it.