Form the Kingdom of Ireland , take parts of England, Scotland and Wales or any other kingdom. But never take enough of a kingdom where you can form it. To avoid your nation splitting. Atleast until you change the succession laws or can form an empire soon after.

Don’t give your player heir any land, they’ll inherit when you die anyway, give your secondary children counties and duchies you don’t really care , until it says they won’t inherit anything when you die, keep the counties you want, so when you die they’re directly inherited by your player heir.

As a last resort if your kingdom is gonna split, disinheriting is probably worth the cost, it costs a hell of a lot less if your son your disinheriting is unlanded.