Do you know anybody that works at a trauma center? ER? Motorcycle riders are called "organ donors".
I know everyone's going to argue with me on this, but the fact is that you are less safe on a motorcycle than you are in a car. Maybe most motorcycle accidents are caused by the idiots/squids riding them, but there are also way too many posts about guys who have ridden for 20-30 years accident free, and then get rear ended by some 19 year old teenage girl who was texting. As much as you want to and try to be situationally aware 100% of the time, sometimes that 0.5% of the time you're not watching your mirrors are when you get hit, and all it takes is one hit to completely change your life.
Talk to him and see why he wants to get a motorcycle? Does he just want to look cool? Does he think that motorcycles make you feel more connected, free, and open compared to cars? Or that it's better on gas (yes it is but nobody gets one because of this), can split lanes (in CA), and provides more of a thrill just driving around town? Figure out why he wants it and figure out how he can get it, without risking his life on the streets. Figure out why you don't want him to ride it. Then, you can come up with a compromise.
Maybe you can get him a track bike, or maybe you can get him a track car and split paying for track days? Maybe there's a way of restricting when and where he can ride it?