To some extent the question of proximate cause will be determined in the criminal prosecution. If Zimmerman were found guilty of any degree of homicide and the conviction stood, he would be in a situation where it would be nearly impossible to collect from Martin's parents. If Zimmerman is acquitted (as I think he should be), then he has been found to be (not innocent as such) not guilty in the contemplation of our law. This COULD mean that the jury found him to be factually and legally innocent or it could merely mean that any guilt was not proven beyond a reasonable doubt (which happened in OJ's murder case).
In a civil action, different jurisdictions have different rules. If Florida is a state where comparative liability is imposed, and a jury decided that the liability for Zimmerman's damages were 75% that of Trayvon (and his parents) and 25% that of Zimmerman, subtract 25% from 75% and Trayvon or his parents pay 50% of the damages. That may be further cut if Florida limits the parental liability for act of the minor to a set dollar figure.
If the Florida standard were one recognizing contributory liability by the victim, then the award of damages might be prohibited altogether.
There might also be counterclaims and setoffs in which Zimmerman might be found liable for Trayvon's death.
Your question had been "could Zimmerman have a case against Martin's parents...?" Most lawyers would say yes, that almost anyone can sue almost anyone. The more important question is as to whether the person suing is likely to prove his/her/their case, and for how much? Another is whether there is a statute of limitations requiring that any action be filed within a limited time period and whether that time has expired.
God bless you and yours!
P.S. I am retired and no longer practice. The foregoing is the opinion of one who is now just a man in the street, one who never practiced law in Florida and one who will gladly defer to any experienced Florida lawyer on the question asked. The foregoing is an intellectual exercise and NOT legal advice.
Thank you. My severely limited understanding had me thinking aloing similar lines.