I'm so sorry to hear what happened to your son. The passage of almost 20 years has surely done little to ease your pain, especially with the killer still enjoying her freedom.
Having said that, I'm afraid I can't be very encouraging.
First of all, any recourse you may have must be through either the Iowa state courts or the federal courts located in that state. California courts would have no jurisdiction, even though you now live here.
Second, sheriffs and prosecutors are not legally required to pursue any particular case. There are legitimate reasons why even some murder cases are not prosecuted. That doesn't mean the reasons in this case were legitimate, but it does mean that in general the authorities have discretion to decide which cases to pursue and which to let go.
If that discretion is exercised in a corrupt manner, your remedy is to expose the corruption and/or to seek the help of a different entity. When you can't trust the county sheriff you can take your concerns to the state or to your municipal police (if any). You can also complain to the FBI, which may not technically have jurisdiction over a particular crime but which can prosecute officials for corruption.
I don't know whether you ever had a viable civil claim against the sheriff, since law enforcement officers are generally immune from liability for the way they do their jobs. Further, even if they were liable they would only be on the hook for the harm caused by their failure to pursue the case -- which, as I explained earlier, they probably were not required to do anyway. And even if they were, they would only be liable for failing to go after the killer, not for the death of your son. Perhaps most importantly, it is hard to imagine that you could still bring a civil suit 19 years later. There is generally no time limit for murder prosecutions, but there almost always is for civil lawsuits. The applicable limit here almost surely passed long ago, though you may want to ask some Iowa lawyers about this.
Your rights against the county itself were probably stronger than those against individual deputies and prosecutors, but such a claim would still be subject to a limitations period that has almost certainly lapsed.
It may still be legal for the state to prosecute the killer, but that doesn't mean doing so would be feasible at this point. It sounds like the sheriff never even gathered evidence or generated much of a case file, so proving the killer's guilt would likely be very difficult. Even if evidence was collected it may have deteriorated by now. Similarly, witnesses may have died in the interim, and the memories of those who are still alive may have faded.
You should press the issue with the current sheriff and D.A. where you used to live. They may be more sympathetic than the authorities you dealt with at the time. If they aren't, you should take the matter to the Iowa attorney general's office and/or the FBI. Realistically, though, you should not be surprised if the killer is never prosecuted. You may, however, at least get some of the answers you have thus far been denied.
I'm sorry I can't be more encouraging.
