I was wondering whether the expected refund was going to be spent on meth or marijuana, but now that you have mentioned Texas we know it's a case of Lone Star beer.  (Probably every day or two.)  

If the guy mentioned common-law marriage to you, maybe that's what happened.  If you mentioned common-law marriage to him and IRS, you're digging yourself into a deeper hole.

If you're concerned about amending a joint return for two people who aren't married, look up the IRS rules on annulments.  

If the kid's story is "we were married," then forget about an amended joint return and have him sign a letter to the IRS auditor stating that he cannot support any of the deductions and requesting that a notice of deficiency be issued disallowing everything.  (The problem with that, however, is that didn't they come up with some phony Schedule C income?)  

If the kid's story is "we were not married," then provide whatever evidence you have of that, along with an original return with single filing status.  Maybe the best evidence is that they did get married, but in a later year.