My primary advice is to have him finance it completely on his own and make it more expensive by mandating safety gear. No matter how smart they are, there's a lot of stupid going around at that age. We all were there and we all were stupid. If you disagree, you're in denial.
I got a bike at 21 years old because that was the only legal/insured transportation I could afford. I had a few close calls. Distracted drivers have made it much more dangerous for commute riding in traffic. If he only wants it for leisure riding out west, it might not be so bad.
If he somehow pulls it off...
One piece of safety gear I recommend is a headlight modulator.
https://www.kisantech.com/view_product.php?product=P115W-D
They go inline with the headlight bulb and are easy to install.
He should definitely take the motorcycle safety course. That course saved my life by teaching me things that I otherwise would have had to learn the hard way. They also teach you to be an amazingly defensive driver.
Start out with a 500cc or below bike. A 250cc is plenty. Try to get a cruiser style instead of a sport bike style.