Diurnal (Day stance) is very similar to WHM and offer similar healing power to WHM with some nuanced differences in application. Bear in mind that without a WHM, neither of you will have a MND party bonus so your healing / DPS will be slightly weaker due to that and thus you may need to add a little bit more healing than what you're used to doing.

It's better for him to stay in Diurnal (Day stance) versus Nocturnal (night stance) because his shields will override your shields and your shields have the potential to be much more powerful than your AST.

SCH/AST works fine as a combination, you both just need to work around the differences. That comes with time and gaining a trust and understanding with your healing partner.