I found this newborn kitten and he was fine up until he became 5 days old. He doesn't seem to want his formula anymore. He strains when I wipe him to go to the bathroom. I figured he is constipated. What I want to know is what I can give him or do for him to make him go?

Submerge his back end in warm water. It should help with constipation. If it doesn't, then you need to get him to a vet, asap. Kittens can go downhill very quickly.