My mother made a will, leaving 5% for three grandchildren,

Hello, My mother made a will, leaving 5% for three grandchildren, and 5% for 2 grandchildren, and for her son 5% and her daughter 40%. The value of her assets are 320,000. After her passing, her son is claiming the 1975 inheritance act to say he wants 2/3, leaving all her grandchildren out of the will. He is on disability allowance and housing, stating he is unable to work. He was fully independant and did not rely on my mother, living elsewhere. He claims he had a close relationship with my mother, but never visited her prior to her passing or went to the funeral. I am her daughter and live in Canada, but she made me POA and executor, and her grandaughter POA also. She told me she only left 5% for him because she was advised by the solicitor that he could dispute it. Please advice us how to go about this and if he has any rights to take away the gifts of the other beneficiaries.
JA: Can you tell me where this is? And just to clarify, what paperwork has been filed?
Customer: This is in England, but I live in Canada, and had a close relationship with my mother. It has not been filed yet. She died March 2019 and probate is completed. He has contested the will with a solicitor in October. We have responded to his solicitor's letter via another solicitor, who has not sent anything forward to them yet. He is from the law firm that addressed the probate and he seems to be very busy.
JA: What documents or supporting evidence do you have?
Customer: the POA, the Will, listing me as the executor and other friends as secondary executor, the transaction of the house being sold, the estate agents etc, the funeral bills and all her documentations (eg nursing home costs, council tax, gas and water bills, all her accounts)...is that what you were asking for?
JA: There is no charge to talk to me. But I'm not the Estate Lawyer.
Customer: Okay, I am really asking about litigation, if my brother has a case. He has submitted his list of costs, such as cost for car maintenance, how much it would cost for him to buy a house and the fact he has not worked for 40 years because of health reasons (no attachment given of assessments done)
JA: Anything else you want the Lawyer to know before I connect you?
Customer: I think that's it at the moment