This is a good question. Unfortnately it is difficult for me to diagnose your son's skin condirtion without being able to see him. What you need is a diagnosis of his skin problem. You have tried usually effective treatment for his dry skin with no benefit to him so it is time to take him to see a skin specialist [dermatologist] in order to get a clear diagnosis of his problem and the correct tretament. Take the moisturisers you have tried so far along with you to show the dermatologist.
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