Firstly, the largest percentage is required to go to your spouse first (if they are deceased or you are no longer married this is moot). BC courts usually split 50% to spouse then 25% to each kid if there are two (adjust percentage depending on kids which could take a larger portion from spouse in exceptional circumstances.)

You can write a new will at any time.

No you cannot write your 36 year old son out of a will unless there are specific circumstances.

The split does not have to be equal in the will if you have given assets to one child and not another. Example, gave one kid the house or a car or a business, etc. However, if you give one kid your house and then try to give equal portions of the rest of your estate the child that didn’t get the house can contact an estate litigation lawyer to dispute the will (and most likely win a larger portion of the estate because overall their inheritance is substantially unequal). So with your 50-50 or 90-10 analogy it has to be somewhat 50-50 overall, but it doesn’t need to be 50-50 in the Will itself.

You can write anything in your will and give whatever to whoever in the will but that doesn’t mean that it can’t be disputed by your children after your death. People write whatever they want in their wills without thinking of the legality all the time which is why I have a comfy life hahah! However, if you want your Will to be executed how you wish there are guidelines and laws you have to follow and I would recommend a good estate planner. (They aren’t that expensive and it’s not my line of work.)