Trotting on pavement is actually good for the feet, in small bursts (2-3 minutes per ride total). It will help harden his soles and encourage the coffin bone to retract up into the hoof capsule, giving you more concavity and shorter feet
You can use your knife and dig away at the dead sole at the back corner of his hoof, by the heel. Dig out any flaky material. Stop when you get to a waxy material. You can rasp his heels down to within 1/8th to 1/16th of an inch longer or above the level of the sole. Don't rasp down to the level of the sole though. If you cut into the heel too much, you'll actually encourage more growth, lol.