Im 25, and my dad has MS and wishes he could play, loves having a go on the gun in his garden loves hearing me talk about it and be so passionate about it (Along with my Pontiac Firebird) my mum says she would like to try it and is relieved because she now has things to buy on Christmases and birthdays because I buy the things I want and that really pisses her off lol.
Explain to him your part of a team (He would understand if it was football, your passionate about something, your spending time with nice people and your not one of the little fcuks out being a tw-at and damaging my car, or trying to be hard on street corners!
I played Rugby for Watford for 5 years and got more injuries playing that than anything I have done since but I bet he would understand you getting hurt for that sport!
Show this vid and he might get it! Made my friends buy a load of kit lol
http://uk.youtube.com/watch?v=_s6H0g2x3Pw&mode=related&search=
Thats my two pennies lol. Oh and as long as you enjoy it and can fund it then keep enjoying it. Lifes horces for cources people like different things. He should just be thankful you actually have a hobby!