You can’t leave your money to whoever you wish in all countries, but in England and Wales you can. If your grandson is a minor your will would have to stipulate the age at which he receives it and appoint trustees to hold it until then. You need to get your will written by a solicitor and they will tell you it is advisable to place a note with your will explaining why you have disinherited your son; if your son is in any way financially dependent on you he may have grounds to contest the will if no provision has been made for him - court proceedings can quickly wipe out an estate..
I would suggest you think carefully about the possible consequences of a young person inheriting money before committing to this course of action. It can end badly. An alternative can be to leave it to trustees to be used for his benefit - eg education, house purchase etc. The solicitor will advise.