What if sources of light affected it's effectiveness?

For example in a dark room, it's 100% effective, but anywhere out in broad daylight or even under moon or heavy starlight is enough to interfere (max 40% full moon, max 20% starlight on new moon), for example. With it virtually impossible under high noon directly below the sun or source of light.

This way he'll be able to use it, but he can't avoid everything completely without moving in normal circumstances.

If it needs more weaknesses, probably limit what he can do while phasing and choose between recovery time (delay after exiting the form) or startup time (delay before entering form).