Can I file a motion to enforce so that my ex will have to adhere to the decree, and add our child to a health insurance policy?

You can apply for CHIPS insurance. If you qualify it is very reasonably priced. Or, You can purchase insurance for the child and then ask the judge to order ex to pay. Blue Cross Blue Shield used to offer a plan for kids for under $150 a month.
I would encourage you to talk to ex and see if ex expects to get a job soon. I suspect that your decree states that if insurance lapses then that party becomes 100% responsible for all costs. So if ex was ordered to pay insurance then the ex should take out the insurance policy and pay for it.
When ex gets a new job, someone will need to have the Employer withholding order sent (cost around $15) so that the employer knows to withhold from earnings.
If ex loses jobs frequently then you just need to add the child to your policy or purchase insurance.
You probably need to hire a family law attorney to assist you.
Unfortunately it is difficult for a judge to make a parent be a "responsible" parent.

Great advice from Fran.
I'll add that you need to prepare for the worst case scenario - immediately. Consider putting the child on the CHIPS program asap. You can sort out the details in an enforcement later. An ongoing dispute between the parents does not help the child if there is a catastrophe.

You need to go the Department of Health and apply for Medicaid as you sound as though your facts may qualify. Alternatively look into any Children's Health Insurance Program. Also look into any private health insurance coverage options. You will eventually file to modify and enforce the court's medical support orders applying for OAG services or with private family law attorney.