What could be a problem

What could be the problem?

My son is 11 year old. Six month ago for the first time he complain on rectal discharge:mucus,color: something between yelow and brown. His main problem caused by discharge is red burned sore around anus.We tried many creams even we tried with medicals against parasites and fungal (Nystatin) but the only things that help is TRIDERM cream. While we apply TRIDERM he dont have discharge from rectum. Also we wisited gastroenterologist, he made some research of ampoul of colon and biopsy everything was ok.Blood test of IgE was result 1684U/ml which refer to some kind of alergic. But we know that he has a problem with asthma and he using in case of need Ventolin,Symbicort etc. I would be very thankful if you have any idea what could be his problem. Did you ever heared something simillar?Also we tried with some correctiion with diet we throw out milk and everything made from milk,ketchup,corn fleakes, but the problem is still there.How long we could use TRIDERM as now that is the onlyone solution.

Thanks inadvance,

Stanislava Markovic