This is a tougher case than usual. If the son was on the lease, and is now in the military, I am not sure how he was served. Sub-service should be ok, but courts are kind of leary to evict military.
But that does not give these people the right to not pay rent. The law is intended to help in most other cases, but not rent. There are 2 capable people that could pay rent. I would want to know why they can't pay the rent. I guess now that they filed an answer, you will be going to court.
I would hire an attorney if I were you, but the judges in these courts are somewhat lenient on formal discovery and rules of evidence. Bring all your documents and show proof of non-payment of rent. The court should rule in your favor. Not paying rent is not an excuse ever, even in most cases if you don't fix things. There are other remedies for that.
This is general legal advice intended for informational purposes only and does not create and attorney/client relationship. If you wish further advice, please contact an attorney of your own choosing or you may contact me for further advice and make other arrangements including retaining my services.