While I cannot provide you with a specific answer as to, "What deems a person voluntary under employed", it would be in your best interest to provide the court with the medical documentation you speak of. It has been my experience that courts are busy and pressed for time so make sure your medical records are in order, highlight the pertinent parts of the record and always present your most compelling evidence first. So you if have a Doctors note stating that it would be in the best interests...
You need to provide more information in your question. Have you already been to court? Is a parenting plan in the works? Are you sharing custody with the children's father? What tests has your boyfriend passed? Who do you want to, "Help out"?
I believe the answer to your question is yes but the bigger question might be what can you do if he/she refuses to go? If that is the case a good old fashion grounding or denial of certain privileges might do the trick; No T.V., No Internet, No Cell phone etc. Especially if you are financially providing for this child.
Since your step daughter lives in Arizona I believe Arizona has jurisdiction over the matter. You could attempt to contact law schools in Arizona and see if they provide free or low cost legal clinics.
Your son is certainly old enough that the courts would listen to his desires. You can go back to court and request full custody. If you cannot afford an attorney check your local law schools to see if they provide free or low cost legal clinics.