You're perfectly entitled to draw up such a will. Upon your death though, the son who's been left out of your will could try to get a court to vary its provisions under the terms of the Inheritance (Provision for Family and Dependants) Act 1975 on the grounds that your will failed to provide him with 'reasonable financial provision' :
https://www.legislation.gov.uk/ukpga/1975/63/contents
(Such an application wouldn't automatically succeed. It would be up to the court to determine whether he had any right to part of your estate).
If you'll be creating a trust to hold the money for your grandchildren until they're 25, you will need to think carefully about who your trustees should be and what, if any, access they should have to the money for the benefit of your grandchildren prior to the trust maturing. (For example, it's not unusual for money to be put into trust for minors and/or young adults but with the trustees permitted to draw upon those funds, prior to the trust maturing, to help the beneficiaries of the trust through their education).
Trusts can be tricky to get right (especially when they don't mature at the age of 18). Although I often suggest that people don't need to employ a solicitor to draw up their wills, this is one occasion where I feel that it would be wisest to seek professional help.