FOR THE WATER QUESTION: Yes! He can get dehydrated iwthout that fluid! If you cant keep it donw, ask for fluid treatment bags. They are easy to use -- your cat can get really sick and harm his kidneys if he doesnt drink. If you cant just get that water in him slowly thorughout the day (adding it to food can help) then do ask about IV fluids, I gave those to my kitty at home two years. Some people are afraid to, but it's not hard and email me for any tips.
AS for food, is it Science Diet AD? As long as you get even a quarter can in him as a start, that's good, cause that stuff is very high calorie. Try to give a little more each day. Make sure you do it when you are not stressed out, and explain to him what you are doing. Hold him still but comfortably, and possibly cuddle first. Mix the food with the water, and only syringe about 1/3 ml at a time. I would give about a shotglass full at most at a time when I had to do this -- even less. And just do a handful of meals thorughout the day to keep the calories in there. If your kitty looks like he is getting sick, don't overdo it, let him go for a while. Also, pepcid AC could be helpful if he is nauseous as he starts to eat again. Don't worry about the full can -- I know you are supposed to do it, but my cat survived on one half can for a few weeks before she recovered and ate again. More hte better, but important thing is to get him to eat. Cats eat differently anwyay. There are also high nutrient and high cal vitamins you can rub on his teeth to help out. Good luck! I know it's not fun, but it will help him recover.
PS, ignore the comment about not forcefeeding a cat. Even if you haven;t gotten to the vet yet, a cat can get terminal liver disease if he doesn't eat for two days or more. It's just temporary and he will appreciate it when he is recovered and playing again. Cats don't ever like to be "told" what to do, so he is just offended, he will get over it. (he may not feel well either, but food in his tummy will calm it down)