The terms remain the same, we still pay the monthly mortgage payment.

Your payments are known. 

Start by serving proper notice of the management change and request payment details.

Your son is responsible for making payments on time or ruining this seller's credit. 

What kind of account or vehicle would you suggest to start saving down payment money for my first rental property?

Unless you've done substantial improvements to the building or used a large down payment when buying, it's unlikely this will work.Perhaps your best move is to find a new primary residence that is 2-4 units and repeat the process.

But let’s go a later deeper - my down payment would come out of a well performing stock portfolio.

I financed this property using down payment funds from 403B and a local commercial bank. 15% down from personal funds. 85% down from community commercial bank. 3yr balloon, 20yr amortization, 7% interest rate.

Quote from @Jack Townsend: I heard David Greene mention a loan product that only requires a 10% down payment on the purchase price and 10% of the rehab budget.

Live in one unit - It's a great strategy when using a bank for financing as the down payment and interest rates are more favorable to owner occupants.