The only people who will have control over your estate when you die are the executors of your will. As long as you haven't named her as an executor (which, from what you've written, seems to be extremely unlikely) she can't have any say in what happens to your money and other assets.
If your son is unable to look after his own finances though, your executors will need to set up a trust for him and, as his carer, it's inevitable that his mother will need access to those funds to look after his needs. The terms of the trust will state that she can only use the funds for the benefit of your son but it's important that there are trustees who can check that what she actually does with the money.
So, as Atalanta states, it might be best to get the wording of the trust sorted out before you die (rather than leaving the task to the executors of your estate).