Personally... I'd be like, why not just call him
William? To me it does not sound wrong with Caemlyn, Huckleberry and
Gilead. It's obviously more usual than them, but... Williams is very usual as well, in that it's heard all the time.
If someone introduced themselves as Williams to me, I'd actually assume it was his last name and that he must be famous and I ought to know who he was!
Or it'd me think of a quote from The
Lies of Locke Lamora: "Your parents must have been misers to give you nothing but a surname."
If you wanted something a bit less normal, there's versions like
Illiam, Willum,
Gwillym... heaps of languages have variations of
William.