If I blocked my opponent so that he was unable to fly off an asteroid does he still have to roll to see if he takes damage?

After he executes his maneuver, if his base or the maneuver template overlapped the asteroid then yes.

If you execute a maneuver and overlap other ships that force to to not actually move at all, you are still executing a maneuver.

Just for completeness, same goes for stationary maneuvers. (got asked about that at a game last night)

Yep, especially nice if the front of his base still hasnt cleared the edge of the asteroid. Rarer because you need to be in the right place with the right shaped asteroid, but still awesome.

yep happened last night in a casual game I just let it slide as that had never come up before. was his last ship anyway and it was about to get blown up..