The best thing to do is to is take a giant scoop of the critical care, add water slowly and mix until it gets to the best consistency. Get as much of that into the piggie as you can and refrigerate the rest. The next feeding, 4 hours later, warm it up to room temperature and repeat the process, mixing more if you need it. And don't worry too much about spilling it. Just make sure you get the sryinge into his mouth about as far as his back molars.
It'll keep in the fridge for 24 hours after it's mixed up. You can microwave it as long as it doesn't get hot and it's evenly warmed up. I have a 100 CC syringe I kept it in and I ran hot water over it until it was room temperature. Either way works as long as it's not too warm. The guinea pig usually gets enough water from the critical care, but you can give him extra water during feeding or anytime you feel he is not getting enough. There's a hand feeding guide on this site that is very informative.
There's a whole bunch of useful information on just about everything you might encounter with a guinea pig. It's indexed, so click on the link you need. Keep weighing him daily. Very important to ensure he's not losing or gaining too much weight. Best of luck and I hope your piggie gets better.