just out of interest ironmonger, have you written to the british or us boxing commissions during your research ?
Not sure how much info they might have, or be willing to give, but a politely written letter, explaining how you're trying to find all you can about yout relative may do the trick.
They must have had documents given/shown to them for his boxing licences, and it's possible some info still exists....it's how much time they have to help you that counts I guess. Got to be worth a couple of stamps though