With a friend it's always better to fall into it than openly suggest it IMO.
People are weird about wanking, and if a guy suggests it to a straight friend, they'll immediately think they're gay, and that is an immediate shut-down, however much they might secretly want to do it.
I would say try to create a scenario, accidentally leave porn in the DVD player the next time you're together and about to watch a movie, then let him find it and see what the response is. Mention a really good porn site you found the next time you're alone and suggest checking it out. "Accidentally" leave a wanking toy out the next time he comes over and see what he says about it (this worked for me recently, and we got to share the toy, one in each end with our helmets bumping and sliding together!)
There are plenty of ways to create the scenario, just use your imagination.
But there are some rules...
1. Avoid discussion of sexuality at all cost.
2. Don't suggest going further than wanking near each other, let them take the lead and make those moves if/when they want to.
3. Limit any shared porn to straight, and bisexual if there's no guys getting it on. Only progress on from this at his lead.
4. If you get to being naked, do not comment on his size/shape in anything less than a complimentary tone. If he's bigger than you, say so and appreciate, but if he's smaller, do not point out that you are bigger.
Good luck.