Our fastest bowler can be a little wayward with the new ball and I think better suited to bowling first change. The captain keeps giving him the new ball thinking he might bounce the openers out, but it rarely happens, and the opposition gets off to a flyer.
Our first change comes on and bowls line and length and gets wickets. Shouldn’t he open the bowling and let the quicker bowler bowl with less shine on the ball.?

The fast bowler should be focusing on bowling a good length trying to hit the off bail and using the short ball as a surprise weapon.
This applies irrespective if he is opening the bowling or first change.
On the one hand the openers are usually best equipped to play fast bowling but on the other hand if the fast bowler can display his pace and get an opener out he can create doubts in the rest of the opposition as a strike weapon.

I think you need to work and communicate with the fastest bowler, to tidy up his line and length. Work with him so he understands what’s expected of him and that he understands the plan.
If he can’t execute the plan, then replace him with the first change bowler. If he is wayward in his first two overs, replace him. The message will soon get through.
The best result is to have your quick bowling the right areas from the start, so it is worth persisting to get him right. Pace and the new ball are always a great combination for early wickets.