I hear what all of the other moms are saying and agree at some level (the coach is volunteering his time). However, here are some options:
My son played coach pitch this past spring and the coach had multiple parent helpers. One of them ALWAYS pitched - I guess because he was a good pitcher. But, the other thing is - the coach divided them up into about three groups during practice. One practiced hitting, one practiced catching/throwing and the other practiced with the kids actually on the field. They did these things for about 15 minutes and then they rotated. So, if your coach is not doing this, you could recommend something like this - in a diplomatic way: like...hey..the kids are pretty rowdy today, why don't we break them up into smaller/more managable groups. You practice with them on the field and I will practice pitching to them (somewhere way in the outfield). Hopefully, the coach will see that they are consistently hitting someone else's pitch better. OR, when the game rolls around, you can say..."Hey..they have been practicing with me pitching to them - how about I pitch this game so you can focus on other areas."
If the coach doesn't "hear" it from you politely, then flat out point out to him that he doesn't pitch very well and you would like to replace him as the pitcher. I know the other moms are focusing on the fact that he is volunteering, but the reality is, there is more to baseball than hitting. The kids need to experience getting on the base and running - learning not to run on a pop fly, etc. There is a whole realm of experiences they are missing if they can't get on base.
I understand that the coach is volunteering and the only option I see is if YOU (or your husband or one of the other parent) is willing to step up and be the pitcher 100% of the time. You need to be there and the coach needs to be able to count on you.
Good luck!
L.