First of all, shouldn't it be your husband asking us??
Secondly, try removing the shower head itself (it unscrews off the
little arm to which it's attached). It could be the head itself is plugged.
Thirdly, if step two doesn't work, take the diverter out and look
inside to see if something's caught within sight. If both hot and cold
comes out of the tub spout, then, obviously, the problem is between the
diverter and the shower arm.
Most diverters like yours have a washer and a gasket. If the gasket
comes apart, it can lodge in the shower outlet, or the shower riser, or
the shower head.
Lastly, if nothing's worked so far, take the shower head off, and
remove the diverter and then blow hard on the shower arm to see if air
comes out from where the diverter was. The gasket or washer could have
lodged in the vertical pipe (riser) between the diverter and the shower
arm. Be certain to do this in private, or you'll be teased to death.
If none of these work, then write back for more advice.