I can't speak to the experience of a teen, but it does take a lot more insulin to bring down a high above 300. A 10.2% A1c means his average blood glucose is above 240, an unhealthy level. There is no direct cause and effect relationship between A1c levels and secondary diabetes complications. What we do know is the risk of complication does go up as the A1c rises. Most doctors will counsel to keep the A1c below 7%.
What I'm trying to say is that some diabetics with in-range (<7%) average glucose may develop complications and some diabetics with much higher average glucose don't. There are no guarantees with diabetes but you can tip the scales in your favor by keeping glucose reasonably closer to normal.
You didn't say about the insulin therapy he uses. Is he on an insulin pump or does he use multiple daily injections with a long acting insulin and a shorter acting mealtime insulin?
Where you likely need the most help is helping to motivate him to buy-in to a program to address this. If he was younger I might suggest a diabetes camp where he could meet others his age that live with diabetes every day. Perhaps talk therapy could help him or a setting where he could socially mix with some of his diabetic peers.
From what I've read, your son is following the path of many teenage diabetics before him. I've read many accounts of diabetics in their 20s who turn things around following many teenage years with poor control.
In any case, I think you could tremendously benefit with interaction with other parents of teenage diabetics. There are many members here with children who have diabetes. I'm sure you'll get some feedback from them.
You're right to be concerned. I hope that your son can be convinced that diabetes plays for keeps, the stakes are high, and the sooner he takes action, the better it will be for him. I hope you can find some support, too, no matter what your son chooses.