Answer: Mr. Fisher did not speak up at the meeting because he did not know which side to choose. If he chose Mrs. Fisher's side he would anger coach, but if he took coach's side he would anger Mrs. Fisher. This would also be the reason why he was squirming in his seat, he was nervous about the argument and how it is going to end up. He didn't want things to get heated between coach and Mrs. Fisher because it will also effect his relationship with the coach but he also does not want this to get in between the "Eric Fisher Dream".
Explanation: (Pg. 58) "I looked at Dad, sitting back on the couch next to Mom. His whole body was stiff, rigid, like he was dead. What would he do? Would he publicly take Coach Warner's side against Mom? Or would he defend her and anger Erik's coach?" On this page, it includes when the meeting was held and on this paragraph, it includes why Mr. Fisher was so silent. I also used prior knowledge of the book for the "Eric Fisher Dream" part.