Check out this article

http://www.breastfeeding.asn.au/bfinfo/lactose.html

Does it ring any bells for you, particularly the last section about lactose overload? Does he have a longer stretch overnight? Do your breasts feel fuller in the morning?

Mucousy poos can be a sign of an allergy or intolerance........but that would usually be every (or at least most) poo. It doesn't sound as though that's what is going on for your DS.