I would not recommend increasing his domperidone dose without the consent of your doctor. Domperidone has some dangerous side effects so you need to make sure you are following your doctors instructions. Is the Ph study the only testing he has had?
He had a Ph study when he was 2 which showed intermittent reflux. We have tried elevating the head of his bed, but he simply turns round and lies the other way round, i.e. head down! We have now stopped his bedtime drink, and have increased his dose of domperidone in the evening only. The doctor said not to increase it, but increasing just the evening dose seems to have improved things.
Welcome to the gastroenterology community! What tests have his doctor run on him? You might want to try elevating the head of his bed to see if this helps.