I am sorry to hear of the situation.
Sadly, although there is of course the law to protect beneficiaries and misdoings by an Executor, in practical terms, it is whether it is financially feasible to take action.
Executors are under a duty to carry out the wishes of the deceased and to comply with the legacies bequesthed in the Will.
If an Executor is acting dishonestly, the co-Executor can make an application to Court requesting that the other Executor be removed from his position. This is of course going to be an expensive job, and you would be personally liable costs if there were not sufficient funds in the Estate to fund the proceedings.
Likewise, an Executor who has acted dishonestly and taken items from the Estate which belong to another beneficiary can be called to account and return such items, if the co- Executor or beneficiary who has lost out takes Court action.
I'm sure you will probably agree the above actions are not practical in your circumstances.
Certainly, the son can not legally ask you to step down.
If I were you, I would instruct a local Wills Solicitor to write to the son setting out his wrongdoings, and possibly threatening Court action if he does not put right what he has done wrong. The mere threat of Court may make the son see sense and correct what he has done.
I am sorry I do not have a magical answer , but the above is probably all you can do in this sorry situation.
I hope I have assisted.