I'm not opposed to that, being given a ring or something to allow me to use my powers.
Regardless of magic object or no, his powers will have the affinity of "shadow". Without much preparation, aside from a short incantation, he can use spells that focus on simple mind tricks, like shapes passing across the corner of your eye on in dark places, or ventriloquism like sounds coming from the distance. By writing a spell on a solid surface, he can cover an area in supernatural darkness, blocking the vision of supernatural beings. With a lengthy ritual that involves scribing a phrase on the people he wishes to hide, he can unnoticeably block the sight of supernatural beings, similar to how normal people cannot see the supernatural. A different spell can make this work in reverse. With a long, repetitive chant, he can bring down magical hiding spots. The stronger the illusion, the longer the chant takes. As such, he could notice a weak spell right away, but a powerful one could go unnoticed without hours of repetitive chanting. By putting a spell phrase on a normal person and reciting a long spell, he can unveil the fog for them so that they can see the supernatural world.
Finally, the most obvious characteristic of shadows is that they take on the forms of real objects, as such, his most powerful ability is Mimicry. After seeing a spell performed, he can do it himself, just not as well. It is just an imitation after all. He can't use it indefinitely, either. The more powerful the spell, the shorter the time frame in which he can mimic it. Similar to how you can see or read about how to do something, and do it yourself, but forget how to do it later.
Would these abilities be in line with the campaign's power level?