First thing I noticed is that you want to use a double 2x6 for your ridge. Some inspection departments require the depth of the ridge beam be greater than or equal to the cut depth of your joist. Normally, a 2x12 will do, depends on your roof slope and is a geometry problem. You will not need a king post , and if you space your roof rafters on same spacing as the 2x10's (remember a 2x10 is nominal dimensions, detail this as 1 1/2" x 9 1/4" actual dimension) then you will have very good lateral support at the top plate, which will control the deflection of the walls due to rafter horizontal loads... Adding collar beams near ridge will allow for a ceiling if desired. The new framing normally would not be an issue because it is just the weight of lumber, shingles, etc. The issue you need to be concerned with is the storage weight. Typically, storage can be a large dead load. If he is putting old clothes and x-mas ornaments don't worry, if he is putting stacks of book and magazines, this could be an issue. You will need to analyze the 2x10's to be able to carry the additional dead load. I don't have span tables handy, but 21' for 2x10's sounds like they are maxed out. You will get additional deflection from storage loading and may need to add some additional ceiling joists. Plus, they would not be able to use the space as a bonus room or apartment without some concern for deflection of the ceiling. Hope this helps. Sounds like you are on the right track !