Do you trust that the coach knows what he's doing? If so, then trust him. It sounds like this new coach has been working with your kid for no more than a month. You may just need to be patient.
My son started playing with a new team a few years ago that he chose because their PC is considered one of the best in our area. This coach made some changes to my son's delivery. For the first couple of months of the season, the boy really didn't look good--not throwing strikes, velo not where it needed to be. But my son kept working on the changes his PC wanted. Then something clicked (at a major tournament, fortunately), and Junior picked up 8 mph in velocity from the prior season and looked great. Things weren't perfect--he had a few outings after that where things weren't so smooth--that's often the nature of these things. But now, after 2 years plus with his PC, he's much better than he would have been had he stuck with his old delivery.