I'm assuming you don't work out at the same gym and that you don't have a private hot tub at your place?
In that case, I'd say alcohol and flattery. If you can get him liqored up a bit and in a one-on-one situation in a private place (your house for example), you could tell him that you're into women but you have this obsession with big cocks. Then flatter him, telling him that his bulge is the hugest you've ever seen in your office and how much you envy/admire him, maybe adding something like it's rare for you to find another guy who is bigger than you are. You can then judge his reaction. If he seems uncomfortable, change to subject. If he's intrigued, take it a step further and suggest some "I'll show you mine if you show me yours".
Another option would be to give him a few drinks at your place and casually ask if he's into porn at all, then offer to show him something really hot you just downloaded and want to share with somebody it's so good.
I've used the alcohol and flattery technique before to convert several (otherwise) straight friends into nut-bust buddies. I've also managed to freak out a few friends, so just take your time and watch the signs he's giving you. ;-)
If he's huge and purposely wearing clothing that exposes this, my guess is that he'd be open to letting you have a look in the right situation. Hell, with some porn, you'd probably ending up stroking one out together on the couch.
Good luck!