How do you know your son is being interrogated? Be very careful in trying to obtain this information directly from your son. You don't want to put him in the middle any more than what your ex-husband is. Also, be careful engaging your ex-husband in emails - sometimes this form of communication conveys something different than what is intended.
The real question is what do you hope to achieve? If your goal is forcing your ex-husband to stop communicating with you in this way, this will be...
Before you make this decision, I highly suggest that you talk to an attorney. You don't have to hire one, just consult. You can talk directly to your ex-wife's attorney regarding changes you wish to be made.
Keep good records regarding your ex's attempts to prevent you from getting the child's medical records. This may not carry the day in a hearing, but if you have other evidence in addition, it will be helpful in future proceedings like a change of custody or a termination of joint custody.
Just the threat of dragging him in to court should (but won't necessarily) make him provide the insurance card. Check your divorce papers (judgment of dissolution and/or parenting agreement) for the requirement of providing you with a card. If your court papers provide that both parents have insurance cards, you can either drag him to court or mediation. Again, just the threat of doing so may make him do the right thing.
You really should hire an attorney. Removal cases are very tough to prove. If he does fail to appear, you can request a default judgment - just make sure he was properly served with the petition. Even if you obtain a default judgment, your ex can request that it be vacated within 30 days.
Check your local court rules, but neither the Illinois Marriage and Dissolution of Marriage Act nor the Supreme Court Rules require a person requesting discovery, such as a financial disclosure, to first disclose his or her finances before requesting the same from you.