Doctors at the University of Notre Dame did a multi year study of the effects of boxing on the brain. We had a yearly boxing tournament called the Bengal Bouts where over 150 students competed each year.
Each fighter took a cognitive ability test before and after each fight. The test measured reaction times and all sorts of other things that would probabloy be afected by getting knocked in the head.
The result? Amateur boxing with headgear and 12-16 oz gloves caused no serious problems whatsoever. Of course there has to be proper coaching and refs. But with proper supervision, amatuer boxing is safe. You can probably find this study on the internet somewhere.
So I say get your son involved if he shows an interest. I wish I got started with boxing earlier. Its great fun, keeps you in shape, and every kid should know how to defend himself.
Just tell him to keep his hands up and not to block with his head.