Will he ever grow out of it?

No, XP is a lifelong disorder.

Is being in the shade enough? What about a cloudy day?

There is still far too much UV in those places for Pacific to be outside unprotected. For scale- Our UV meter gives off a reading of several thousand in full daylight. On a cloudy day, we still get a ready of over 1000. Through a regular window our reading may be around 100. Pacific must be in UV levels of under 10 at all times. No amount of UV is known to be safe for him though, so we usually keep him in spaces under a level 2.