The normal small amount of material on a lawn is not thatch. Thatch is
a dense thick layer that most lawns do not have, so it;s unlikely he
needs to dethatch.

I agree that spring is fine to aerate. Not sure that fall vs spring is
that big of an issue.

For spring fertilizer, I'd wait till about 3rd week in April, then
apply a combo fertilizer/crab grass preventer. If you do it when the
forsythias blown, that is about optimal. Then fertilize again in early
sept, them mid-oct. Test/adjust PH as needed.