I think he will get the current kings lands, at least a part of them. He'll not stay unlanded, at the very least.
I've just done it recently with a unlanded female claimant, and she got more than the capital, but I'm not sure if she got the additional counties when she became Queen or if she simply revoked them.
She'll auto-revoke one (as you pointed out, she can't stay unlanded and be queen). This is the same thing that happens if you take a King's last personal holding but vassals still have land. All holdings beyond that will have to be taken the old fashioned way, but a new king or queen is sure to get plotters and rebels, so it's only a matter of time before the new ruler ends up either unlanded or in a much stronger position...