My son is also a goalie. To wait this long into the season to make this change really doesn't make sense. Are the 2 goalies similar in playing style? Is the team making a playoff push? Some coaches go with the hot hand if they are. Is one a senior and not coming back so the coach is giving him more ice time?
My son, now also 18U, had coaches over the years that use a match up system vs every other game system. The coach that did match ups knew after the 1st couple weeks of practice how he wanted the goalies to play. If we played a fast team that moved the puck around my son played because he was more mobile in the net. If the team was a blast a pray type of team the other goalie started because he was big and took up more of the net. He guaranteed 1 game each if we had 2 in a weekend, if only 1 game it was strictly match up based.
The flaw your kids coach has is not telling them ahead of time who is playing. My son's old teammate at 16U refused to play on a Sat because he wasn't given a minimum of 24hrs notice to mentally prepare for his start. He thought he'd get Sunday's game instead, my son played both games that weekend. My son said a goalie should always be ready to play but some need the notice.
Using practice is flawed and he only had 1 coach at bantam try that approach. The coach learned quickly that the goalie that gave over "110%" at practice was not good in actual games at all, over 7GAA with a save % under .800. To be honest my son doesn't always give 100% at practice but always shows up at gametime, around 3GAA with a save % just under .900 that season.
Hope this helps a little, but probably not.