He rarely has a solid stool and his stools are horribly stinky.
Hi, don't be sorry for the info - soooo much can be told from our LOs stools, and it's amazing how much we all scrutinize what comes out of our LOs bodies.

So - if his stools have always been this way, there is a chance he has an intorlerance/allergy to milk. It could be the lactose, or the milk protein itself. The best way to tell is to eliminate it (which you've done) and since you are seeing an improvement that's a great sign. The thing is since you've gone to Lactaid milk, but also dropped the yogurt and cheese, it could be either one. If there is a milk allergy/intolerance lactose free won't really help. (that's the milk sugar, not the milk protein - both which can cause issues).
Yes, I would absolutely speak to his ped about this. I would ask to see a ped allergist and have then test for the top 8 just to be sure (milk, soy, wheat, eggs, peanuts, treenuts, fish, shellfish).
Hope this helps some. How is he doing now? It can take several days to work it's way out of the system, but if you are seeing slight improvement after 24/48hrs after removing the food, that can be an indicator that there is an issue with that food.