Janini, my DD was like this over last winter but also refused milk and got quite dehydrated.

My suggestions would be...continue in a relaxed way to offer formula / milk and water from different cups / bottles.  When he takes a bit, praise him lots and hopefully he'll like that and catch on!  Have you tried a straw?  Some kids seem to like that because it's a bit more like a game.  Also, I guess you will be offering a range of food textures so if you need to, give him nice sloppy fluid filled stuff to start with (that way you know he's had some) and then finger foods.  Jelly, icecream, custard, fruit puree, thick vege soups are all good option.  Also whatever the organic brand of baby food is does a couple of fruit gels in jars.

If you are worried about dehydration I would check with your GP because the more reliable way to check it is capillary refill time (how quickly the blood refills empty capillaries) - they can show you how to do it properly.  Lots of the other signs don't necessarily point specifically to dehydration so can be misleading.

Good luck.  I didn't really find an answer.  When she was able to hold a bottle herself it stopped being an issue.