I'm 76 years old and have been boating for over half a century so in this case, at least, I know whereof I speak: The old man who is selling his boat because the kids are gone is often akin to the mythical old maid school teacher who "only drove her car once a week to church on Sunday" before she traded it in to the used car lot. I first heard that story in 1951 when my dad fell for it about a Nash Rambler he ended up buying. Since then I've heard both of those stories a number of times over the years.
I'm not saying the "older gentleman" in your case is lying but I can assure you that we old codgers be just as deceitful as anyone else when it comes to selling damaged goods. If he lied when he was young he will more than likely lie when he is old but will have had a lot of years to practice being convincing. So all I am saying is proceed with caution.
When I was looking for a boat last year, I was amazed at the number of twenty year old motors I found that "only had a few hours on them". Apparently most people buy a boat then park it in their garage or carport, never using it, until they decide to sell it twenty five or thirty years later.
As far as letting the seller test the motor for you, remember what Ziggy said: the seller has motivation to lie. If possible get some third party who knows boat motors to test it for you - even if you have to pay for it.
Actually you probably should have someone who knows the ins and outs of boat construction and maintenance check the boat for rotten wood, water soaked flotation foam, etc. before you even consider buying it.