Not knowing what's wrong makes me hesitant to consider assist feeding as it's called on my support list for one of my cats. I'd call the vet, ask if they can work you in as an emergency - I realize it's now late afternoon, but unless they close at 5, you still have some time. Perhaps they could take in for an overnight - get an initial exam done and begin testing.
If it isn't possible, you might want to consider getting him to whatever emergency/after hours clinic this vet recomends for supportive care tonight.
If none of the above is doable, the most I would try is some plain baby meat (no onions in the ingredients), dab this on your finger and gently place some of this in his mouth (be careful not to get accidentally chomped on). If he's drinking, you can mix nutrical in water (makes it a funky white color, but then he can just drink it down). Most pet stores carry Nutrical - a high calorie supplement paste. Another idea re liquids - boil up some chicken - make a plain, unseasoned broth and see if he'll drink this.
But my vote is to get him seen tonight if at all possible. Poor fellow...please keep us updated?