Both my children have CMPI. It takes a while for the gut to heal after cutting out all dairy. 3 weeks minimum i think? He may also be intolerant to soy which is in a lot if things so watch for that. Make sure you are getting fat in your diet so your not losing weight. Nuts, avocado, etc. You can make lots of sweets from scratch and there are a few milk alternatives to try. Also Arnotts Chocolate Ripple and the 100s & 1000s biscuits are dairy free.
I know it's hard but keep going with it - the mucous is a sign something isn't agreeing with him i think?