Leave the science out of it. If he joins he will have plenty time to figure it out then.

Just ask him, don't you wish you could have got shares in facebook before it became what it is today? Well, get your arse to Steemit and you will be able to earn shares in something that just might be worth billions and billions in the future.

Be honest though.. Tell him it could also end up worth nothing, but let him know that most here believe otherwise.

Finally, let him know that if Steemit does go mainstream and becomes the next big thing, he will be known as the first sportsman to jump on it way before it took off, and it will give him the appearance of being ahead of the curve/trend, which, if marketed correctly, will make him an excellent target for sponsors and he will be able to demand a very high price.

That's what I would say... But, I feel this could potentially be good for Steemit if this guy ends up doing well in the UFC, so I will resteem this post to give more people the chance to come up with some better ideas.