Puppy won’t eat!

Hey guys so I’m wondering if anyone has went through the same thing as me. I got my black lab puppy at 8 weeks from a breeder. He’s perfect. He had no problems with food at all. But with his stool would constantly be loose and then it randomly went back to solid. I had to get his second round of vaccines and so i fed him his normal 2 cups in the morning and went to his appointment. He got all his shots and when we came back I gave him a cup of food (which he never has lunch) because they told me he is slightly underweight. That night he didn’t eat, he just slept (from the shots) and the next morning he wouldn’t eat breakfast either. Which was bizarre. I noticed his arm was blown up from the distemper shot and I called the vet and they said it’s normal and to bring him in for pain meds and nausea so he can eat and not be in pain. So I did just that. His arm swelling was gone but he still wasn’t eating. At that point it’s been a couple of days. Since his vaccines he would only eat a 1/4- 1/2 cup a day. I took him to the vet (day 5 now) spent 8 hours in the lot freaking out, spent $800. They did X-rays and blood work. Everything came back normal. His organs are fine, no blockages. They gave me an appetite stimulant, it worked for 2 days and now It’s not again. I tried switching the brand of food several times (none of switch cause diarrhea it’s still solid) I tried home cooked remedies. The only dog food I haven’t tried is Royal Canin. but for human food I’ve done, rice chicken ground beef (he liked the ground beef) chicken broth bone broth cottage cheese milk eggs yogurt peanut butter. He went to my dads while I was at work and my puppy didn’t eat his own food but ate my dads dogs adult food. He ate over a cup. More than he will ever eat of his. The doctors don’t have any answers, so I guess I’m asking. Anything I haven’t tried? Any vitamins? Should I do royal canin? Vet keeps saying he’s extremely picky. But my boy needs to eat adequately.