You must be so proud of him.
Anyway, as somebody who works in a workshop with double basses, I feel it's my duty to tell you about the existence of half sized Double basses meant for children. They are smaller than a normal upright bass but are tuned the normal way. Your boy looks like he must struggle to reach the lower notes on the upright.
This man is playing a half sized bass, showing how much smaller they are than a full sized bass, it'll be perfect for your boy
Maybe you ought to get your son one of them so playing one won't be as hard on him.
Just some friendly advice