Well then, it's really a matter of picking your poison if compromises have to be made. But I have to ask an important question....... which is paramount to you, lighting your childs' face properly or shadows that show behind him?
Let's rethink this.
Its going to be kinda a harsh light rock and roll theme so i'm using speedlights with no major modifiers. I have a snooted one coming in from behind stage right, and one with a just a diffuser comming in from the front stage left.
You don't say what kind of diffuser, so I'm guessing a pop-on dome diffuser directly to your flash head. Let that be your fill light. Place it pretty close to lens axis an above the camera. This will be used to fill in the shadows created by your main/key light. So now you have another light and a snoot. Do you have any other light modifiers? Cause if you use the snoot, it will be very directional lighting and only spotlight your son. So with that, I'd suggest to use the flash bare if no other modifiers are available. Set the flash head to a fairly wide zoom and 1-1/2 to 2 stops brighter than your fill. Also place the main/key 30° to 45° camera left or right and angled ~45° down. Of course, the further away from your subject, the harsher the light (and shadows) will be because it will be a smaller light source. You may als want to feather the main/key light so that 1) the hotspot isn't directed solely on your son and 2) it will hit the background to help light it
<got a phone call, still talking... she won't stop talking....have lost my train of thought>