Elimination diet. W/ the diarrhea, it's almost certainly a food allergy. I would start with cutting all forms of dairy (read labels carefully) out of BOTH of your diets for a minimum of 2 weeks and see where that gets you. While you're at it, cut out all peanuts, nuts, and shellfish. Those should be avoided anyway, even if he isn't allergic, until he's at least 3. If you already don't do dairy, eggs and soy are next on the list.
Unless you have a gut feeling about what food it could be or have recently added a new food. wheat and/or gluten are common, corn, sesame, fish, etc. In that case, cut whatever food you have a feeling about. Minimum time to see definite results is 2 weeks, may take up to 4 to be completely clear. You should see some results within a week, though, and if you don't, it's either not tht food, or it's that food AND another food.
And also start some good dairy free probiotics.

Seeing an allergist can be helpfull if you find a good one, but testing is not accurate in young kids.