I hear you, @ZZpop
Just to clarify, at home outside school days/hours your son usually has 1:1 or 2:1 care, provided by you and/or your DH, except for a small amount of overnight away respite? While your DH is in hospital 2 carers are being provided outside school days/hours but you are also being required to stay at home then, meaning that the care ratio is 3:1? Is that all correct? If so, and your son has been assessed as needing 2:1, then you should be able to go out if you feel your son will cope with you not being there - perhaps he will need some time to get used to the carers and they him - and if you have trust in the carers.
Of course it's not at all unreasonable to want to visit your DH in hospital and to know that the level and type of your son's needs mean that it is not in his or anyone else's best interests to take him with you. I understand that you would usually plan to visit DH while your son is at school but during the holidays this will not be possible if you are required to stay at home as a third carer. I hope that, with support, you are able to get this requirement reviewed and dropped.
Being pragmatic about it, would you intend to drive to hospital and does your son travel in the car ok, perhaps in the back seat with the support and security of a Crelling safety harness? If he would be ok or even enjoy the drive, could you get the agreement in place for the carers to travel with you and stay in the car with your son, with some snacks and toys for an hour, while you go in to visit your DH? This has worked for me, albeit with the help of a trusted friend rather than paid carers. Would friends or family help you by doing this, if the carers won't? Even if just once a week it would be something, alongside daily virtual visiting.
The treatment of unpaid family carers in the UK, but especially in England, is something the government, services and society should be ashamed of. It seems that the will and money is there to pay - often private, profit-making companies - for severely disabled children and adults to be hidden away in sometimes completely unsuitable and abusive secure hospital and residential settings for months and even years on end. If unpaid family carers received timely and proper support at home, this could often be avoided.
All the best to you and your family, OP 