He may be looking for some oral input. Does he have an OT? They would be able to get you some ideas or come up with a sensory diet. Some things you could try would be yogurt, chewy things, electric toothbrush (use the back of the toothbrush and rub it on his cheeks),. My son will be 6 in a month and he still licks a little bit, usually when he is over stimulated.

Hello< This is what worked for us ...I would try to use a small amount of vinegar,, Get one of those squirt ketsup bottles, you know like the ones in the resturaunts....

When your son goes for the "NON LICK ITEMS" say "NO" and when he continues to do lick that item.. then squirt a bit of vinegar in his mouth or lemon juice, and repeat "NO" ACKIE!! He is continuing to do behavior that is for sure sensory seeking, I would give him a plastic container full of toys he CAN go after,put stickers on it inside I would get bumpy rubber balls, smooth rubber ducks and items you think he will gravitate too.. this way, you can wash off the "GO to Bucket" in the bathtub with him at night time, that can be his GO to bucket, al the time put a small one in the car too perhaps, and when he does go after the items in that bucket, big ATTA BOY ! reward him, with a sweet treat like a tootsie role or an M&M... just like potty training, re- enforce good behavior. Hope it is useful ! let me know if it works

Parasites, can be treated with antifungals.

Could be parasites. More common than you think. My son was doing the licking thing, clicking his tongue, etc. We got a negative stool test but decided to treat anyway... And sure enough...

There are some sensory toys/tools out there you could try. There are some specifically for the mouth, and there are some for fidgeting he could carry or wear like a key chain. I saw a catalog at my son's school but I don't remember what it was called. An OT can help you formulate a plan. Ask if they deal with sensory issues. Not all do.