Tips on how to hold maximum water to increase stroke rate

First Post here but love the forum and some of the experts are amazing. I'm looking for tips on how to hold maximum water to increase stroke rate/ Propulsion. My son went 1:03 on a 100M butterfly last year with 48 strokes and now he goes the same time but with 45 strokes. He has worked hard on his technique and it shows that his distance per stroke has improved but his stroke rate is slower. Which is understandable but if he were to improve his stroke rate, he would get improvement in his time. One of the coaches told him to hold as much water as possible and increase his stroke rate without sacrificing his technique. The question is how to do it. I found few articles, ie focus on the underwater pull/catch but more specific advise on how to hold water will be greatly appreciated.