He typically uses mostly pentatonic scales, though sometimes using modes of the major scale in his more melodic solos (november rain, sweet child of mine, etc.). He will sometimes also use harmonic minor. In Sweet Child of Mine the second part of the solo switches from the key of E to D harmonic minor. The last part of November Rain is in harmonic minor as well. Oh, he sometimes uses the blues scale too (added flat fifth to normal pentatonic). Hope this helps...