I really wish I could help but this is outside of my field of practice. I don't see patients under 13 and haven't dealt with neonates except my own for 10 years.
I hope you find the support you need and will keep you in my thoughts and prayers.
If you live near a major metro area you can probably find a pediatric cardiac unit. It is very important that he see a specialist who deals with neonates with heart problems, instead of a general cardiologist as disease which effect children don't always effect adults and vice versa.
If you live near the greater Milwaukee area they have an excellent cardiac unit for children at the local Childrens Hospital.
Are you using a slow flow nipple, sometimes this can help because if the infant is feeding to fast with a regular nipple then the infant can forget to breath (apnea). I suggest talkng to your pediatrician/cardiologist that gave you the holter monitor or whoever does the follow up appointment. Some helpful hints might be if your noticing this happen with the infant falling asleep then try ways to keep awake during feedings like play with the feet, hair, bright light, have some noise and do not allow to get to comfy that the baby wants to sleep.
I would contact the NICU that discharged him. They should have some follow up suggestions or services. Maybe having a home health nurse come in a few times would help.