Typically a trust is the solution to this. You set up a trust fund, and either fund it with money now, or (more likely, in your situation given your income) make it the beneficiary of a life insurance policy.
The trust would invest in something with a reasonable return; say assume 3% to be safe. So you'd need £24,000 per year, so you need at least £800,000 capital; let's say an even £1 million would be better (that way if there are a few lean years your son still has plenty of wiggle room to draw off of capital). Obviously more is safer, and means you can use safer investments; £500k would be enough if you assumed a 5% return, but that requires more aggressive investments and more risk.
Trusts also are probably taxable, depending on various details (I don't know UK law, in particular), so you may need a bit more to take taxes into account.
You'd need to set up a trust (which incurs administrative and legal expenses), designate a trustee (hopefully you already have a will which designates a particular person as guardian for your son if you and your wife both pass away; you can either designate this person trustee, or someone else, depending on how you prefer to do things). Make sure the trustee(s) you choose are reliable and will have your son's best interests in mind; you can certainly write the trust documents in such a way as to require that of the trustee(s), but it's very hard to enforce such a provision, particularly after you've died.
In the US, we have a concept called an Irrevocable Life Insurance Trust; that is one possibility for you, if the UK has the same concept - this is a trust that specifically exists to be the beneficiary (and, technically, owner) of the life insurance policy. From a quick google search, it looks as if the UK does have similar concepts, but please consult a specialist to get the complete information, and to set it up.
Also, don't forget to take into account your other son. Until he's finished with school, his needs should be considered as well; either in a separate trust with a separate life insurance policy, or perhaps using funds from your pension or savings. He's not in a much better position than his brother if you pass away when he's 9, after all.
You'll also want to make sure that he understands why this trust exists, and why his brother will presumably inherit substantially more than he will, regardless of when that actually happens. Being open about financial decisions is important, especially when on face they appear to be imbalanced. It's the sort of thing that if you discuss it, and the reasons for it, over the course of your son's childhood, he'll be much more understanding when you do eventually die, than if it's a surprise to him.
