In the end, you have to decide on your own if it's something you want to do and if feel it's worth it.It's not an easy choice.

@Bryan Richard  You might be able to refinance into an alternative loan product.  

While I thought I would be able to get financing from an alternative lender, it hasn't come through and now I'm considering private lending at 9%. 

Personal debt-to-income ratio is not a consideration for underwriting these loans.In the alternative, some people go to private lenders (in simplistic terms, groups of wealthy people who have pooled together to form a mortgage company) who are willing to lend on investment properties that banks are not willing to underwrite.

Please advise with tips or suggestions, alternatives. 

If you're an aggressive investor there are literally hundreds of choices each month for apartments syndications and crowdfunding site deals.

Is this choice ethical and legal?

The bank is asking for 20% down investment in a duplex. How can I get around this for my first investment.

Alternatively you can obtain clear title after 20 years of using the property if you do not have "Color of Title" (if you had mere possession and did not lease from the son or pay him any portion of the rents).Color of title would be a deed from someone (the son) who you have reason to believe had the right to transfer the property to you. 

Alternatively, I could sell the property.