If you get the DBX Comp 4 there is a good chance that he will use it as you can custom fit it to make it as comfortable as possible.
I’ve had one for 4 years and it took some fettling initially with the various spacers et al before I got it fitting correctly, but I now wear it all the time without it beiong a hindrance whatsoever.
As andymac mentioned above, worn incorrectly though and it can be more dangerous than not wearing one!! With no helmet on you MUST undo it, even stood around or just walking. Helmet on and you MUST have the brace on corectly.
Personally I think it’s the comfiest brace out there, I’ve tried an Alpine Stars one and that sat on my collar bones and hurt like hell.