If he is that badly dehydrated you can soak him in luke warm water (should almost feel cool to your hand or the same temp as your hand), just a shallow tub, or even in the sink, just a few centimeters deep so he doesn't feel the need to try and 'swim'. If he is very weak and 'slouchy' don't leave him! and support his head if he cant hold it up out of the water, he will probably try and drink the water which is good, do this for about 10min every day until he is drinking on his own... dont go the syringe route if you can help it, as this is very stressful for him, see if he wont drink his bath instead
Any old spotlight should work the main thing now is keeping him warm! just make sure it doesn't get over 40degC like Buck said...
You can try and tempt him with a little piece of chicken liver... if he is not interested in eating keep up the hydration routine and try again the next day... he should start eating on his own when he is properly hydrated.
Good luck with him! keep us posted!