I don't know what the mucusy stool is. Both of my boys had/have food allergies. They were diagnosed within months of being born (oldest was 3 months and youngest was 1 day old. He was diagnosed early because they have such extreme allergies that they knew what to look for). Their symptoms were:
extreme fussiness during and after eating
explosive diarrhea usually during feading or right after
excessive bm diapers~more than 4 a day
excessive spit up~ my oldest would spit up all of the time constantly. It will also smell horrible.
If I were you, I would try the elimination diet. If it is food allergies, he will feel and be doing a lot better within a day. If it continues, it has to be something else.