I am sure he has this "loan" buried in a corporation of some kind. People like this never have "personal" loans they keep everything in a business at arm's length. Remember the rich have many accountants and lawyers structuring situations with many layers ... Trump is the prime example.
Now for your situation ... personal interest has not been deductible for decades. And MARGIN interest "may" be deductible on the 4952 but with restrictions (of which a big player would be able to make use of if structured properly). AND maybe you could use the tracing rules but ONLY IF you could make a case in an IRS audit that supports that the interest was solely for the purpose of investing in a business property.
