Hi nitoda
Migration from DLA to PIP is supposed to start again in 2028 - but the DWP has not said anything about those still left on the list recently. We can't tell you what will happen if PIP becomes a gateway to the UC Health Element, but I assume they'll realise they have to do something. As you said, your son will not be the only person affected if it does indeed happen.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems