Found a stray 6-7 week old male kitten. How can I teach it to use the litter box? Get along with other 2 cats?

I have a 3 year old male, fixed, all paws declawed male and a 7 year old female, fixed front paws dewclawed cat. My mom and I found a young kitten outside and took it to the vet, where it was given a good health check. We've decided to keep him but he doesn't want to use the litter box, we've been taking him outside to a hole our outside cats use. Any ideas on how to train him? Any one also know how to get the older cats to quit hissing at him when he walks by?