How does your son feel about his choices? If he is happy, it is important that you do not pressure him into applying to universities and colleges that you prefer. It is ultimately his decision, so let him make up his own mind, but you can help him to see the positive and negative aspects of his choices.
If he is unhappy, all of you need to get together with his school or college and discuss the best way forward. If he was wrongly advised, his school could write to us to ask to substitute his choices. If he just made errors he now regrets, he could add further choices if he has not already used all five choices. If he has used five choices, he could turn down any offers he has received which would make him eligible for Extra. Extra is a chance to make a further choice. Later in the application cycle, Clearing offers the same opportunity to those who are holding no offers.