At least the diarrhea stopped. Give it until tonight, if he hasn't pooped by then, give him an apple. Plus talk to him and let him know that the apple is good for him and will help him poop and feel better. Only give one slice to start out with, because apples have natural sugar in them it wouldn't be a good idea to give to much and have the diarrhea come back.
Harley may be constipated, not sure though. That is why I say to give it until tonight to see if he poops before giving him any apple. The apple will help him poop. I did this with Sweetie when she didn't poop all day the first time she had constipation and she pooped after eating the apple. But give it until tonight before giving him the apple and only one slice at a time.