I'm sorry for your loss, OP, and realise that it must be shocking to hear that what you thought would happen with your mother's house is unlikely to happen. It's the last thing you need when you're grieving.
The link helpfully posted by @ZadocPDederick says quite clearly that
"If the applicant is a surviving spouse or civil partner of the deceased, they can claim at a higher maintenance standard which is whatever is necessary for their maintenance in all of the circumstances. This is significant because with spouses and civil partners, the court can take into account the standard and style of living and reasonable expectations of the applicant, including what they might have expected to get if the relationship had ended with divorce instead of death, and that may well exceed what they need for their day to day maintenance."
As your mother had the higher income, he could clearly show that she was supporting him financially. And had they divorced, he almost certainly would have a decent amount.
I really think you need to see a solicitor, ideally the one who did the will. And I think you should try and get your head round the idea that he will get something, and possibly something quite substantial, and that it will be in your long-term interests to try and negotiate with him rather than to get involved in costly and expensive litigation.
I'm so sorry this has happened, and your anger is perfectly understandable. But it's also not likely to be helpful in sorting out the estate.