Accel. Dev. is very important to 400m runners in the sense that if you can't properly accelerate and get yourself up to speed in a race you won't be able to gain any sort of momentum the 2nd 100m resulting in slower times. Also, it means you'll have to work harder to run faster.
As far as the hills are concerned, I think a heavy dose of both would be appropriate. My only thing is again, whether he is going short-to-long or vice versa as that would play a big part in what he'd be doing over the summer.