Whether or not the young man understands the significance of the Eucharist is irrelevant. In some circumstances, the Church can and does give Communion to those who do not understand its significance. What is a problem is that this young man's physical condition hinders his ability to swallow Communion properly, leading to its inadvertent but real profanation when he has spit out the host and drooled in the chalice. (Not to mention that the drooling causes a health hazard to others who receive after him.)
I can only recommend that the pastor meet with the young man's mother, kindly explain to her again why her son need not receive Communion, and then gently inform her that the parish's clergy and EMHCs will be instructed not to offer Communion during Mass to her son. (If Communion can be safely given to him in private, her son might be brought Communion by an EMHC to the homebound.) As a pastoral gesture, and assuming that this is done in his parish, the pastor might invite the woman to bring her son to a priest or deacon for a blessing during Communion. If the pastor chooses not to act on this matter, I recommend writing to the bishop and explaining the situation to him.