Firstly, I'm a sister, not an expert, so do make sure you go to Ask the Expert as well. That said, I'm happy to share what it really would have helped my parents to have known at an earlier stage.
1st, on benefits, there's a savings limit below which they are ignored for means testing. I think it's £10,000. You need to check, but unless you have a lot in that savings account you won't need to worry. More importantly, if your son doesn't understand money, somebody needs to be able to help manage it for him. I don't know whether or not he has "capacity", reflecting how much he is able to make his own decisions. If he is able to understand what he's doing, and is willing, he can sign a Power of Attorney, allowing you to manage his money for him. If he doesn't, you can go to the Court of Protection to be appointed his legal deputy for finances. If he can understand enough to give a Power of Attorney, that's easier, but in either case you need someone to sign that he does, or doesn't understand. The forms are on a government website, with lots of explanatory notes, though a solicitor, can hand hold if you need it. I'm guessing your son will also need to claim benefits. Someone can be an appointee to deal with those. It needs to be done separately from the POA/deputyship, but things can get very complicated if it's done by different people (I know from experience!). It's not a good idea. Some social services have legal departments that can take over responsibility, or can refer you on to a solicitor. Obviously there's a charge, and not a small one, but that can be worth doing if you're not comfortable with managing it all yourself. There is, though, a lot of advice online and, when it comes to claiming benefits, the Citizens Advice Bureau can be very helpful.
Having mentioned Power of Attorney, or Court of Protection, you can apply separately for responsibility for welfare. You may not realise that your rights to see medical records, authorise treatment, even decide where and how he lives, will disappear once your son becomes 18. Again, I don't know your son's particular circumstances, but you may find a gp unwilling to discuss anything with you without consent. As an example, my brother couldn't be given blood tests, as he would have needed to be restrained or sedated, and that couldn't be done without consent.
With regard to your will, it really is worth paying to talk to a specialist solicitor. I don't think there's an easy answer, and a solicitor may know of any court cases that have tested the law. My understanding is that the advice my parents had still holds good, but please go to the experts. As far as I know, you are right that, if a share or your estate goes to your son, it will disappear, going towards his care. Unfortunately, if you try to get round that by splitting it between your "regular" children, the will can be challenged as you won't have made provision for him. The usual attempt to get round this is by setting up a discretionary trust. There would be a list of possible beneficiaries (in our case, children, grandchildren, their spouses etc), who could receive money from the trust at the discretion of the trustees. Your son would therefore have no absolute right to the money, but it could be used for his benefit. It could also be taken as making some sort of provision for him. I'm told that there is a risk that this could be challenged as well, but is the safest option available. Our trust was set up at the same time as my parents wrote their wills, with a tax free NS&I certificate, so that it could just sit dormant until needed. The one warning I would give is that running a discretionary trust has its own complications and costs, so you will be leaving a bit of a headache to your other children, but, unless a specialist solicitor can come up with a better solution that may be something that just has to happen. We're still sorting out my mother's estate, and, after talking to the solicitor, will be taking most of the assets out of the trust. I'll keep them in my name, but separate, so that I can use them for my brother - it's just simpler. We're not expecting any challenge, but our situation is slightly different, as my brother had a stroke not long before my mother died, so is now funded by health, which is not means tested - so there's no point in social services challenging it.
Sorry, for the long post. You should probably be able to research and sort out managing your son's money by yourself, and with help from the internet, but you really do need to find a specialist solicitor for the wills. Most of all though, the changes in their responsibilities/rights to have a say in his care when my brother turned 18, took my parents completely by surprise, so , important as money is, it's worth taking a look at the position there as well.