You likely won't be able to just flat out stop a behavior like that, it'll need to be replaced with another more appropriate manifestation.
I'd offer up a counter productive behavior to reroute what he's conditioned himself to do. Meaning, when you catch him get him to do something that won't allow him to use his mouth in that way.
Because you say he's got some diagnosis's I'd say something repetitive like singing a nursery rhyme or even just a letter sound should do the trick. When you catch him ask him to sing you the "one two buckle my shoe" rhyme. Or to mimic some silly nonsense phrase like "burple purple twerple!"
For some kids making faces works well too.
"Hey Johnny, what's a lizard/snake say?" then demonstrate tongue flickering. Or even engage him and ask him to show you a happy face, or a silly or scared face.
Odds are there is a process that leads to it, whether he's under or over stimulated in the moment, tired, sensory seeking or whatever. You can most likely rewire his response to that if you are consistent. What's interesting is paying that close of attention to it for awhile you might be able to pin point what's leading up to it and intervene there, and it'll be mighty entertaining if you do manage to rewire the response as you'll start to witness him either start randomly singing, or coming up with other nonsense phrases or making goofy faces unprompted