It is hard to know specifically without knowing the player well and knowing his swing.
In general, "streaky" players tend to have more moving parts in their swing, more timing mechanisms. A simpler approach to mechanics can lead to more consistency.
How is his eyesight? I'd get that checked. Again, speaking in generalizations, I'd make sure he'd work on the mental part of the game a lot. Does he get overly frustrated when he goes 0 for 4 in a game? Does it start to get in his head "oh no here comes another slump."? It is hard enough to hit good pitching, add in a defeatist attitude or less than 100% focus and that makes it even more difficult.
If it has nothing to do with eyesight or his mental state, and his mechanics are fairly simple and sound, how is his approach at the plate? Is he chasing bad pitches? Not crushing fastballs early in counts?