with the head/body ratio, and head shape like that no need to check for anything - that is a 100% boy!
how big he is right now? most staring croaking at 5.5", but some do that earlier or later, but have no doubts he will do that you can play him youtube video of other pixies calling, sometimes they respond. usually they call more during breeding season or when barometric pressure outside is changing. When I hear all my frogs croaking a lot - 100% guarantee it is going to rain/snow soon.