oh Megan, I had to laugh when I read the title of this post! Cute..
Anyways, how bizzar. I'm not sure what to suggest. I think it could just be his body adjusting to solids. Poo can take on many forms once solids are introduced. Are you offering a little "drink" after/with solids. Maybe just some expressed BM in a cup, or a little nursing to top up? Just to make sure his little belly is well lubricated in digesting the solids?