Our 7yo (autistic, lots of delays including speech and understanding) has a history of refusing to drink when unwell. He's got a viral infection (coughing, fever, croaky throat) and has refused to drink anything at all today.
I think I should bundle him in the car and get him to A&E for fluids now but DH says it'll stress him too much.
Fwiw we have tried the syringes, ice lollies, jellies, juice - basically all the things you're going to suggest. All of it has been firmly refused, and we can't force him. He's too strong for us.
I think leaving him overnight is dangerous. DH thinks it'll do more long term damage to take him than good.