I understand your being upset about all this.. But really there is nothing you can do..

Maybe you should sit your son down and explain to him your situation in lack of monies that you would leave for him do to your situation and maybe he will question his father.

I know here in the USA people can file petitions regarding who is left what money .. Its a long drawn out process and it really does typically follow the will of the deceased, unless it can be determined that he was not in his right mind when he made his will ... Not sure if there are same type of legal actions where you are.

I know you just want to protect your son but you cant expect that his father is going to do X things.