At least it wasn't at the age of 37. For his 58th birthday get him a voucher for a tatoo parlour to add the caption "My Dear Pony" underneath it. Let him live the dream OP.
And this affects you...how exactly? Is there some reason he SHOULDN'T have gotten that particular tattoo? Do you have something against tattoos? Or against unicorns? Perhaps you'd rather he get "MALICE" tattooed across his forehead?