Autistics and Mysophobia or Germophobia

OCD's are more common in Autistics than most other folk.

In this case they likely stem from <associations> and <anxieties>.

There are three vital steps ahead of you.  They will bring much greater benefit to his life (and yours!) than merely resolving germophobia.

Many famous people who had this OCD (Mysophobia) were marvellous for all of humanity... because wherever sensitivities exist, properly focused and directed, they deliver for the person concerned and all humanity later on.  A magnifying glass can be used to burn... or to see better.

First, asking what you can do about his phobia is a question that needs changing from a self-focused (but not selfish) "what can I do" to a "why does my son" question that seeks knowledge, not cure, because when you find the knowledge... any cure becomes unnecessary, the matter resolves itself.

Second most important thing to consider is to never dismiss your son's concerns, or seek to 'cure' him by aversion 'therapy' (punishment, loss, pain of any type being applied), exposure 'therapy' (seeking to deaden his response mechanism by progressively greater and greater coerced submission to germs), or behavioural 'therapy' (programming, conditioning, manipulating).

Third most important thing is to find a psychologist who is neuroaffirmative (ask them if they accept and respect minds of all possible types) who regularly help people with OCD's... successfully.  Ensuring they do not use the three techniques above that cause a higher harm, TRAUMA, by overlooking your son is a human, not a robot or a crossword-puzzle to be programmed or solved.

They might for instance radically review stressors and anxiety-ramping factors in your son's life, as these can overspeed thought-processes and get processes stuck along the way.

They might look a decision-making powers and choices in your son's life to see if they are at an appropriate high-enough and flexible-enough level for where he is at, as loss of power and control can lead to misplaced attempts to control the unseen or unnecessary.

They may employ or recommend ways to increase your son's early scientific knowledge, like understanding of how some microbes are our friends, use of a microscope, scientific safety and sterility protocols, and how robust most of us are in the face of attack by the invisible world of bacteria, macrophages, viruses, fungi, and prions.

They may help you look at adjacent therapy... not the direct contacts involved in vegetable gardening with soil etc, but with growing beans in cotton wool in a glass jar, hydroponics, terrariums, things adjacent to dirty-hand contact... things using gloves and limiting contact severely.

They may investigate tactile sensitivity or TACTILE DEFENSIVENESS, as sometimes it is not so much a phobia or fear of germs, as a phobia or fear of real or imagined touch or tactile feel.

Make sure they do not use drugs, CBT, DBT, aversion, exposure, ABA as treatments.  Ever.

It is not a germ issue.

It is not a rationality issue.

It is much deeper than irrational fear of germs.

It is about fear/fright followed by fight, flight, fall, freeze, or fawn responses.

It is about control of sensory processes that can flood a very different mind.

Respect his body and brain.

Mixed in with the phobia or whatever, is something very precious, huge.

Best wishes.