Please don't attempt to do it yourself. I used to write wills for a living but there were several occasions when I would refuse to draft a particular will. This was because the potential client really needed to seek the services of a solicitor who also had expertise in financial planning. Your case would be one such situation.
Suppose you fell under a bus next year. Your son wouldn't get the money for 25 years, so what would happen to it until then? I assume that you'd want the money to be earning interest but who should decide how it shoud be invested?
Your will needs to create a trust. It then needs to specify who the trustees should be, together with any other specific requirements. Unlike drawing up a simple will, this is a complex area and needs the services of a solicitor.
Chris