You described the typical "loin shot" that your son made on that doe. They drop instantly due to the shock to the spine, then get up and walk off. You will find little blood.

The first rule of muzzleloading, after you take a shot, RELOAD. Maybe he would have had time to shoot that deer again. I have lost a deer the exact same way.

I have seen good bloodtrails with FPBs. I generally aim to break the offside shoulder if the angle allows. I also shoot about 1/3 up from the belly on broadside shots, so blood does not have to fill the cavity before leaking out.