Some of these suggestions definitely show that not too many of us remember how it felt to be 20 years old and invincible.
When I was 21 I was in the police academy. I smoked at that time. Every day, we ran five miles. And like clockwork, my PT/fight instructors would dog me daily about smoking, to the point that one of them used to show me photos of diseased lungs and tell me how I was going to die a horrible death. And not a bit of it made a difference to me. I was 21, I was in great shape, I was still running a seven minute mile, and the fact that my instructors kept on my ass about smoking on a regular basis was a huge reason why I wouldn't quit, even though at the time I was probably down to about 5 cigarettes a day (that's all I had time for). All the horror stories about cancer, all the yelling, had the exact opposite effect on me than was intended, because I was a 21 year old hardhead, just like a LOT of 21 year old men. I don't know the OP or his brother, so I don't know what sort of relationship they have, but I know that if I was my 21 year old self and an older brother started getting in my face demanding that I do something, and showing me videos of motorcycle accidents, and putting his foot down and trying to drag me to someplace I didn't want to go, and wouldn't respect the fact that I'd already said "no" pretty clearly, we would eventually come to blows.
He's a 21 year old man, in site of what our current culture says. Maybe to most of us who are on the other side of 50, 21 is still a kid, but for the purposes of "fightin', screwin' or dyin'", he's a man. Treat him like one. Put your "older wiser brother" hat away for a little bit, sit down with him, man to man, and be honest with him. Tell him you've seen him ride, and he's not ready to ride on the roads yet based on your experience. Tell him you're concerned for him, because it's dangerous out there, even when you do have skills, and that he's in even more danger because he's not good enough yet. If he goes down that "well, you didn't take the course" road, then tell him you wish you had, and that you'll take it with him. And if he won't listen, then leave it be, unless you want to damage the relationship you have with your brother. Or worse yet, drive him to do something out if spite that he might not have done otherwise. Even when you're way over 21, being nagged about something repeatedly still isn't a good tactic. How many of you, when you hit fifty, started getting the "colonoscopy" speech from every family member/medical professional/concerned friend whose Dad died of colorectal cancer? And how many of you, even though you knew that colorectal cancer was the second deadliest cancer and kills about 270,000 people in the US a year (because people would quote those stats to you endlessly), would still keep putting off the colonoscopy? And would start rolling your eyes and making roto-rooter jokes every time someone brought it up?
"No matter where you go, there you are."