It is so hard to know! Some do just take longer than others, but you would hate to just leave him when he needs help... I guess it's your call but if you wanted to VERY gently pull away some shell right around where his hole is, then let him rest and see if it encourages him,and then slowly...
I have had ducklings hatch from the wrong side or middle as well. If he is still chirping I wouldn't do much at this point. Give him a break, he is probably worn out from working so hard, it can be quite exhausting, and I know mine always took lengthy breaks after the first major pip. Just...