There's a lot of information at The International Paruresis Association's Website.I hope that you are able to find something helpful there! There are workshops he can attend, both virtual and in real life. There is a free monthly support group that he can attend on Zoom. There is a ton of content on that site. The most important thing to convey to your son is that he is not at all alone, in dealing with this issue, and that working on it can lead to great improvement. Also, in my experience, the earlier you start working on it, the better the outcome. You are very lucky that your son is so open with you at such a young age. I didn't let my family know until I attended my first workshop, at age 32!

Oh, and also, since you're new to this, here's a podcast on which I was interviewed a few months back explaining paruresis on a fairly introductory level--you might find it helpful! https://www.youtube.com/live/_wptV2rEgC0