I'm 31 and still do (put just about everything in my mouth, for sensory reasons), more or less. Of course, I'm sneaky about it.
Either super sneaky ((my mouth is rarely still... my jaw clenches to greater and lesser strengths, running my tongue along itself, my teeth, the curves in my lips, etc... small movements that most people would never notice)), blatantly sneaky ((I cook, and so I'm tasting and snitching all the time)), hedonistically sneaky (( running fabric over my lips, feeling the cool cold edges of a glass bottle and intentionally clinking it against my teeth and playing with it with my lips or the tip of my tongue; breathing with my mouth just slightly open when I'm near something so that I can taste the scent of it; lingering over things that are "allowed" in one's mouth... from swirling fine wine to spagettios)), bad-mom sneaky ((cigars, cigarettes, pen caps)), I really need a 3rd hand sneaky ((I'll stick almost anything in my mouth when my hands are full... truth be told I could just wait 3 seconds and have a spare hand... but my mouth is convenient, and it serves 2 purposes... the 3rd hand and taste/texture/temp)).
Some might say oral fixation, but I'm adhd... it's part of the sensory schtuff that goes along with it. I do other things equally, well... bizarre maybe, with most of my body... and they're all sensory seeking. From walking in (at last count) 14 different and distinct ways, to how I sit, to how I stretch my hands, what clothes I wear, sheets I sleep in, breaths I take. The vast majority of which have been carefully calculated to be next to invisible. As in other people don't notice. But it's taken years. Decades to be precise. How to constantly be getting my sensory "fix" without looking "weird".
Everyone with adhd has sensory stuff. (But not everyone with sensory stuff is adhd, squares & rectangles.) One of the reasons that people used to think we grew out of adhd is that we learn how to be covert. We're still doing highly age inappropriate things... we just figure out ways not to call attention to it.
So the only piece of advice I can really give, is to give him a challenge: figure out a way to get the fix without looking __________ (strange, weird, creepy, whatever... fill in the blank).
But yeah. Just about everything has been in my mouth. Upside? Killer immune system.