My dog is constipated and in pain.?

I have a 4 year old pit and before you guys go off about why I havent taken him to the vet, well today is friday and they close at 4 and I got home at 6ish pm. My dog is super lethargic and can barely walk. His backside is like a cat ready to attack and he doesn't seem good at all. I quickly got him more water which he already had but hes been drinking water like crazy. Hes really wet down to his neck in slober and water and his saliva is all over which is why I think hes constipated and maybe a little dehydrated but hes drinking so much water I dont understand. My vet opens tomorrow morning at 9 am but I cant stand seeing him like this. He also tries to go but he cant and he cries and moans. Oh yea and hes throwing every so often and its all liquid because he wont even eat. He hides behind the bushes and just stays there. What could it be and is there anything I can do ease his pain. How am I suppose to sleep the night knowing hes outside in pain. I think it might be that he gnawed some of the fence off and theres long pieces next to the fence which I know it was him but Ive read that with wood and sharp points he would go more and bloody. Hes never chewed off the fence before so I think something was up that made him want to get out. Please help me. I am going to take him tomorrow but theres no where I can take him at this time.