first some clarity - how similar is the new course? Will he be able to use the same personal statement or will he need to rewrite? Has he really changed what he want to do or is it just that he feels he needs an excuse to change university?
Secondly why has he changed his mind? If this is about perceived "prestige" that's not a good reason to switch. If it's because hes nervous about making friends you can provide reassurance. If it's financial concerns and wanting to be able to live at home you need a different sort of reassurance. If it's a girlfriend he wants to be nearer then the relationship is unlikely to survive the first year of uni, although I doubt you'll get him to believe that.When it is a significant relationship distance doesnt matter.
My own child changed their plans drastically and did a gap year. It seems to be working out well for them. A couple of friends have had children drop out and restart a course elsewhere, that's an expensive option and if they are undecided an extra year may be wise - just make sure they have a good plan for what they do in the year.
He can be released by his universities, they don't want drop outs so will probably let him go. Then he can try extra or clearing but he has to do so knowing he may face a gap year.
If you want to name the universities/ courses it may be possible to find out if his new choice had places in clearing last year. No guarantee they would have ths year but it might provide a clue.