What grade is the adenoids. It should be big enough as you are saying that he breaths through mouth in sleep.
Observe his sleep , if he is not able to breath and suddenly chokes and gets up during sleep it means that the nasal block is more and it is called sleep Apnea. this is not good for the child.
If such episodes have been observed during sleep then it advisable for surgical removal of adenoids. Already he is suffering since 13 years!
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observe child during sleep and if he is choking during sleep it is not good for him.