I think you're doing very well not downing the whisky yourself .
Unfortunately, the feeling dizzy/headache are probably signs of him starting to get a bit dehydrated but at least he's stopped having diarrheoa.
I don't know what else to suggest because I'm sure you would have tried everything - does he understand the consequence of him not drinking anything (i.e. another trip back to the hospital), that might be worth a try (please discount if you've already gone down that route).
I'm sorry you're not getting any help - sounds very hard on a day to day basis. Are the school working with you to get him back because having him at home all the time must be doing your head in!