Just tell him. He'll scoff, but eventually he'll get the point. My dad is like your's but he's 90 now, and he knew when he had to cool it, sometime around 85-86. Gave me his 944 at 89, when he knew he'd had enough
These guys have a completly different frame of reference than you or I, and there is absolutely no way we'll "get it" fully. OTOH, they did what they did for their kids, and they will respond, just not immediately nor will they acknowledge why.