That's interesting what you say - he prefers the back to the front, because I'd imagine that for a lot of people, the reverse would be true. Relinquishing all control can be a tough nut to crack - my boys hated it when they tried it.
Fboab sometimes bemoans the fact that she has "nothing to do", but generally uses the excuse reason - "I won't fit" as justification to not try reversal of positions.
With you on the front, and he on the back - you're likely to need both of you to have feet down when stopped. Even if you're strong, there's the whole stand over height thing at the front - if you're to support him at the back, you'll need to place your feet wider apart and the tubing may not permit that. We never even tried that - we're both fixed-gear riders, and have never had the "need" to freewheel as part of the getaway, so we naturally both do the fixed-gear getaway. Of course, that teaches you to always make a "positive" getaway - there's no room for dithering!
If he's stronger than you, you might feel a surging from the back. When fboab is putting in a stint to help me out (happens a lot lately) there's definitely a feeling of surging from the back. Whether changing the pedal phasing would help that, I've not investigated.
ETA: Who has the gear controls? Hills are just about gearing and speed, and shouldn't affect him at the back - although I can see that if he's fidgety, then at slow speed that might be destabilising.
