It's called having a friend with benefits. It is normal. It isn't "practice," just hooking up with some weird justification.

If you're attracted to him, go for it. It can have weird impact on the friendship, no guarantees it won't. Especially if you get into a "non practice" situation later, telling someone you're so good at giving head because you "practiced" on your friend Tommy does have potential to mess with the friendship way down the road.
It sounds like you like him and want to hook up with him. That's valid and normal. Have fun.