Wow...he seems awful thin. My foster is also 20 weeks but weighs 46lbs.
Like I mentioned in another post I just went through this with my foster and the similarities are unmistakeable. My vet also thought it was quite suspicious that he wasn't vomiting, well hydrated, still very playful and had no blood in his stool however it was very mucousy.
Every time I put him back on the chicken and white rice diet the diarrhea got worse. After we explored every medical possibility my vet recommended I change his food to a different protein (prior to that all the food I was feeding him was chicken/poultry based) as it was possible that he may have an allergy to chicken. When I switched him to the lamb and rice diet his stool was miraculously normal and he has been putting weight on since then.
I would say he eats about 5-6c of food daily in a series of meals.
Maybe try changing the protein before going to raw. If it is an allergy to the protein you will see results very quickly and a bag of food cheaper than a vet visit!
Good luck!!