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help me please

my youngest lad is having difficulty settling in to an anchor point that stays the same he shoots a recurve bow 20# at 28 inches however only draws about 22/23 inches
for christmas santa bought him a new sight and thats where the problems started prior to xmas he was drawing to the corner of mouth fairly consistently and was scoring well on a good day at 9 metres

new sight and he has been told to draw to under his chin which is where the problems started his head is up to much his anchor point is up as well and he wont listen to me (see another post in coaches paradox)
wat im asking for is if he draws to the corner of his mouth still is he going t be ok at further distances which is not wat we are being told
he is being told he has to draw to under his chin and he had a bad day at the club session and i want to bolster his confidence again
btw he is only 8
many thanks in anticipation