I am very sorry for all that you are going through.
The attorney's failure to secure a necessary defense witness could certainly be considered in the determination of his effectiveness as counsel. In as much, if your son is convicted then he may be able to claim his attorney was ineffective and receive a new trial. This would be done through a post conviction proceeding for ineffective assistance of counsel.
Your son has a constitutional right to effective assistance of counsel. That standard does not mean he is entitled to the same resources as the state, nor is he entitled to attorneys who are as experienced as those representing the state. Again, he is only entitled to effective assistance of counsel. To fall below effective requires a substantial departure from the standards of practice in your area. In 12 year of criminal trial practice (literally thousands of cases and nearly 100 jury trials) I have only seen one successful post conviction. While I am not in Florida, the standards for ineffective assistance of counsel are articulated in Strickland vs. Washington, a United States Supreme Court case.
All is not necessarily lost on this witness issue. If the state had him under subpoena and your son's attorney relied upon that subpoena (very common) then the witness may be deemed unavailable. In the case of an unavailable witness, often a judge will permit a previous statement or deposition to be admitted in the absence of the witness. Since the witness would likely assert his Fifth Amendment right against self incrimination at the trial, your son's attorney may have had to take this step anyway.
It is also a remote possibility that his attorney could move for a brief continuance of the trial while this witness is located. I have seen judges adjourn a jury trial for a week or more in an effort to effectuate service on a missing witness. I do not know if this would be a good strategic move, but it is certainly something to consider.
Please reply if I can help further.