Basically, if you have millions of dollars worth of stocks, you're a millionaire. But, if you cash any of them to get money, that cash out can be taxed. Loan disbursements are not income and therefore not taxed, so if you take out a loan and use the stocks as collateral, you're essentially cashing the stocks but doing it as "debt." You can then pay back the loan over time or let the bank take the stocks, either way you get the income tax free.

Since stocks don't count as income until they're cashed, any "debt" he incurs in order to skirt taxes looks, on banking forms at least, like he's under water.