Ok, hate to leave another comment on here, but I feel like if he's been your best friend for 10 years, you should be able to just say "Hey, remember when we jacked off together? Want to try it again?" and it not ruin the friendship. All he should say is "Yes." or "No bro, I don't think that's a good idea" and then you move on.
Me and my best friend jack off together I'd say once a month give or take, and it started with me asking him. I was nervous as hell, but I felt like he IS my best friend, and I feel like I can tell him anything, and he cares enough about me to not let me asking ruin our friendship. We pretty much talk about everything, so this wouldn't be that our of sorts.
So one day I just point blank asked, "I'd like to try jacking off with another guy, and you're the only guy I feel comfortable doing it with. Would you like to do it? Be honest." Obviously he said yes, and it was no big deal.
I guess the question to ask is, if he really your best friend in order for this to ruin your friendship? Seeing as you guys did it in the past It would be weird for him to just end the friendship over you just asking about it? Does anyone else get what I'm saying?