My husband watches our son while I work.. I work typically 3:30-8pm with some breaks in there to relieve him and work from home, but my husband finds things to do.
He goes grocery shopping or to home depot or runs errands. He takes DS to the Rochester library to play, to the park (even when it's a bit colder), Kiddie Klub (after 5pm it's $5) or you can look for coupons, he could probably find playgroups too to hang out with.. maybe look online for Dad's groups?, perhaps even just finding 1-2 other dads that are staying home and get together with them, Dinosaur Hill I think has free programs, the Troy nature center has periodic things.
If you have some expendable cash you could have him check out gymboree, Kindermusik, gymnastics.. they have some parent tot classes for that age so he could meet others parents too. (I'm biased to KI since I teach the classes, but I take my son for gymnastics too).