First of all our digestive systems are not meant to digest milk and dairy products. We lack specific enzymes and that is why so many people have problems. Milk and milk products are mucus producing--that is why he is having problems. You will notice that once you cut dairy out-he most likely will not need his allergy meds. Cows milk is made for baby cow consumption, not people.
Second do not feed him soy. It is terrible for human consumption with the exception of edemame or miso. No tofu either--these products are processed. Soy mimics female hormones in the body and causes migraines, early on-set puberty, cancers. etc...
Rice or almond milk is the best bet. Personally I would choose organic rice milk.
You also want to get him on a really good probiotic--this will help tremndously!! If you need help with any more of this info let me know.