One, you and your husband should review his collective bargaining agreement closely. Many CBAs expressly incorporate the Family Medical Leave Act. If so, his termination would apparently contravene the CBA as well as the FMLA itself.
Two, in addition to pressing his union to take his grievance to arbitration if necessary, he should consult with a local employment lawyer to ascertain his potential judicial remedies under FMLA. OP, based on your responses, it would appear your husband has a potentially solid FMLA claim.
At bottom, an employer covered under FMLA cannot utilize FMLA covered leave -- up to twelve weeks of unpaid medical leave to address "serious health conditions" for himself or for a spouse or a child -- as the basis, in totality or in part, to discipline or discharge a worker. Based on what you have written, OP, this appears to be what his employer has done here.