I'm so sorry, OP, this has been such a shocking betrayal for you. Do take some time for it to sink in before taking any action, and consider counselling, it can be so beneficial at a time like this.
I would strongly advise NOT telling him that you know until you have, as MN always says, got your ducks in a row.
The starting point in divorce following a long term marriage (which yours is) is to split all assets (house equity, savings/investments, pensions, and of course debts) 50:50, and no spousal maintenance.
Child maintenance is usually payable to the main carer until the child is 18, or through f/t education.
However, in cases where one partner (usually the wife) isn't able to work full-time and/or DC need long-term care, there can be a different % split, and spousal maintenance (as well as life long child maintenance) can be awarded.
You need a good lawyer to present your case in such a way to get the best outcome for you and your child.
As well as gathering all the financial info, you should also consider any medical info re your son, and what if any long term care he might need from you. For example, would he ever be able to live independently, or will he need to live with you long-term? Can you get a formal (medical?) assessment that would confirm that?
All of this will affect the financial settlement you agree as part of your divorce.
Be prepared for your H, this apparently 'lovely' man, to become someone completely different when his pension and share of the house is under threat.
Be prepared for him to say it's all his money because he earnt it while all you did was stay at home watching TV and going out with friends (which is what mine said, and many other husbands do too).
The Court will not take any notice of this, by the way, but it can be very hurtful to have one's contribution as a SAHM, especially to a SEN child, so casually dismissed.
To sum up:
- say nothing yet
- gather info
- get a good solicitor
- get some counselling
Good luck 💐💐💐