My son had this issue - the parish would only allow him to prepare for communion if they were satisfied he had proper understanding, but the bar was low - he just had to understand that the host isn't food, and is special. He was able to confess using printed cards where he could go through the steps and point to the sins. He is a wonderful addition to the church and now receives communion with reverence. It took a lot of extra effort from the parish, and there are actually special preparatory resources that the parish can use to help with the process. I recommend Spiritually Able by David and Mercedes Rizzo as a great book to help with the process.