My dad was in a similar situation when he retired from chemistry research. He tried a bunch of things, some of which stuck, some of which didn't but still passed the time and got him into outside interactions. In no particular order some of them were: tutoring maths to adults, tending a community garden vegetable plot, being the treasurer for the local naturalist society, research for a local history group, joining a walking club. IMHO your dad's new hobby should be to keep trying new stuff.