I normally have the kids attempt to connect and then go into to the usage setting and select blocked.
You should then see what URL’s are being blocked and for stuff like microsoft or gmail, etc there can be a lot of them because for some reason they like using heaps of different URLs. when your son tries to connect and it fails look at the blocked URLs and then select each one and select allow. this should put it into the filter for that mode (primary or homework).
It might take a few goes, but that has worked for me getting my girls on the schools google and office365 accounts in homework mode.