Worried about new kitten!

I rescued a kitten from the middle of a busy intersection yesterday! Luckily he is fine and only has a busted lip! He appears to be about 6 weeks old...I have 4 other cats so I am pretty positive about his age. He has worms and fleas which I know I will be able to take care of through treatment, but he is constantly thirsty!! He drinks very often and alot when he does drink! He seems perfectly fine and was wondering if this could just be due to being outside for awhile without fresh water? I know that he was an outside cat because he Will NOT use a litterbox which is another issue I am having! I have put his feces in the designated box I have made for him which is very low to the ground and has litter in it, but he still would rather poop on the floor and pee on my bed! Any suggestions??

For the general health & excess thirst: has he been thoroughly examined by the vet? If not, he should be, right away. He may just be dehydrated, or he may have a condition that needs treating. He needs drugs & blood tests.
   
He was probably born on a city street & has no idea what a litter box is for. Try putting some normal potting soil in the litter box & see if he uses that. He may be familiar with dirt but not cat litter. If he starts using it, you can gradually mix in cat litter in increasing amounts until he's using all litter - & he probably won't notice that.
   
You should confine him to a large crate or cat carrier with his dirt litter box & a sleeping box until he learns to use the litter box - and he should be kept separate from the other cats anyway until he's thoroughly checked by the vet, and all his mites and bugs treated, as he could pass them to your cats.
   
Put all your cats on Revolution For Cats, which kills just about everything that can infect a cat.