It's sensory experience and do more sensory activities - look up on youtube and search sensory activities to redirect him - youtube just google and also can listen to music!!!

Oh he’ll listen to music while doing it, I’ve noticed the more I ignore it and not react he’ll do it less

My boy used to spit on windows and spread it all over with his fingers. Luckily he grew out of it

Spitting in autism is often a form of communication, sensory exploration, or a manifestation of frustration, rather than just disobedience. It is categorized as an aggressive or challenging behavior used to express needs, seek sensory input, or gain attention. Management involves identifying the trigger (function) via ABC charts, using visual supports, positive reinforcement for appropriate behavior, and ensuring consistent, calm, and neutral reactions to minimize reinforcement of the behavior.

Key Reasons for Spitting Behavior:
Communication & Frustration: Children may spit to express anger, request items, or escape tasks when they lack, or are struggling with, verbal communication.
Sensory Seeking: Some individuals may spit to explore sensations in their mouth or to experience the sensory input of the action.
Medical/Physical Causes: It is important to rule out underlying issues like excessive saliva, swallowing difficulties, dental pain, or, rarely, seizures.
Learned Behavior: If spitting has previously resulted in getting attention or a desired outcome, it may become a habitual, learned response.

Strategies for Management:
Identify the Trigger: Use an ABC chart (Antecedent, Behavior, Consequence) to track when, where, and why the behavior occurs.
Functional Communication Training: Teach alternative, positive ways to communicate needs (e.g., using PECS or vocalizing).
Neutral Responses: Minimize attention to the behavior (no yelling or scolding), as this can reinforce it. Immediately and calmly clean it up.
Reinforce Replacement Behaviors: Provide high, positive reinforcement when the child behaves appropriately or uses a replacement communication method.
Sensory Alternatives: Offer safe oral stimulation, such as crunchy foods, chewy toys, or blowing bubbles, to meet sensory needs.

If the behavior persists, it is advisable to consult a behavioral therapist or a specialist who can conduct a Functional Behavioral Assessment (FBA

https://onlinelibrary.wiley.com/doi/full/10.100...
https://www.youtube.com/watch?v=KW8T4fP18Zo

My son grew out of it. Could you get him a squirt bottle or water gun or spray bottle? Maybe he likes the water and you could teach him an alternative. My nonverbal 15 year old did this but outgrew it. I do see it come back if he has dental issues.