We are new to this and we are trying to figure out a solution to this.

Talk to the son and ask for some guidance on how to mutually agree to an appropriate solution going forward.For example, if the son will be making all payments with his money, then it may make sense to have the lease in his name with his mother as an occupant.

A Much much easier solution... and much much cheaper too!

Monitor what happens with this one, though.

Your job is to provide a solution, not make the seller's situation worse. 

A common solution is a Series LLC: basically one mothership LLC with compartments (Series) for each property.

Persistence, trial and error, etc. as well.IMHO, “success” always involves a custom solution for each individual’s journey.

What would you all recommend as the best solution for getting one tenant to move out so that we could move in and qualify for the FHA loan on the property?

Premium might be high because home is vacant so it's not being monitored for any harm but it's imperative to protect your newly purchased asset. 

That's a low hanging fruit.Having said that, I do have someone working with the Chinese government to figure out a legal way to get the money out but that's a long term solution right now.