I understand that there's a reasonable suspicion that he MAY be the killer. Therefore, the police should arrest him and charge him for 2nd degree murder (voluntary killing without any design).
He can, at least in the US, appoints himself as his own attorney (representing yourself).
Now, there are lots of ways to at least confirm that he is not a murderer.
First, if there's a way to trace back magic source or somehow verify if a person is the one casting the spell, then he can bring forward his assistant to testify that she casted a spell on his neck tie last night that was designed for self-defense.
Second, as for a prove to show that he is UNABLE to cast a spell, notice how magic can be used both defensively and offensively. This means that it has, to some extent, the ability to take a life of someone else. Notice how even something that's designed for non-murder related purposes (e.g. vehicle) is also regulated if used in a crime activity (e.g. vehicular manslaughter, reckless driving, etc). In addition, something that's SOLELY designed for self-defense / killing (e.g. firearm) is regulated in law. This means that there must be at least one title that solely dedicated to talk a lot about the punishment for crime committed with such items (e.g. armed robbery, negligent discharge, etc). In fact, for firearm alone in the US, there's a whole section, if not more, describing in details what you cannot do. Considering magic, if exists, is at the same level as firearm, it's impossible for it to not be regulated in even one article of the law book. This is a good evidence that magic really does not exist and thus he cannot use what does not exist.