Legal Question in Wills and Trusts in Massachusetts

No, you cannot make changes to your husband's will.

While you cannot make changes to your ex-husband's will, you should look at your separation agreement to see if it specifically addresses estate planning for the children. If it does, it may be possible that the will would be in breach of that contract. In that case, you might want to bring the matter to the court's attention.

No you cannot prevent your ex-husband from disinhereting your son unless it is part of your separation agreement. You can examine the separation agreement for anything that he is obligted to cover. You and your son can file claims against his estate for any monies due and unpaid under that agreement when he dies.

Your son can always challenge the Will on the grounds of undue influence or that he changed his Will for another prohibited purpose under MA law. He cannot do that until his father dies.