I have sons your age.

I got them road bikes a few years ago, thinking we'd ride together on the paths and maybe on the street when they were older.

But they're not really into cycling, and I'm happy that they each have their own interests that they're as passionate about as I am about cycling.

And to be honest, I'm a little relieved that they didn't get hard core about road cycling, because there are risks to it.

If either of them said they wanted to sell their bike to get another style bike, I'd be fine.

The way you explained it above seems very straightforward. If your dad normally pretty reasonable, then I think just explaining it like you did for us should be OK.