I just noticed that my kitten has a clump of poop entangled in his fur & he's been crying an awful lot I'm worried he might be constipated. Is there anything I can do on my own without having to go to a vet because I currently don't have a car & it's really late.

Can you get him to the vet tomorrow?
For tonight try to keep him quiet and comfortable. If you can, try to remove the poop from his rear end.
How old is he? What kind of food are you feeding him?