I remember my time in university and early 20s as a time of trying lots of jobs out.
Some of them were absolute duffs, some were super-risky, and some were boring as heck. Others were surprisingly fulfilling – and I learned so much from each and every one of them.
I learned what I enjoyed (some of which surprised me), what I didn't enjoy, how to manage tough situations, how to set boundaries, how to make the best of crap periods of time, where my limits were, how to negotiate, how to communicate better, how to hold myself up as an adult... and how to make better decisions about what I signed up for and what I didn't.
My parents could have told me all the 'facts' of these things, and I wouldn't have learned any of it properly that way.
You can't learn to swim by someone explaining it to you - you have to get in the pool.
This period of life is all about experimentation and falling on your face – and I'm so grateful my parents gave me the room to balls up and find my way through. I have no doubt I'm a much better person for it now.
This period of life is all about experimentation and discovery - the good as well as the bad. And that's just as tough for a parent as it is for the young adult, but it's all valuable.
Tell him what it would be smart to consider before he decides, tell him you've got his back 100%, be there if and when it gets rainy and confusing and frustrating, and let him know how proud you are of him for working it all out.
Sounds like you've got a good kid, and you're a good parent for being concerned. Be proud of both of you, and let him find his feet.