Are the bottom of his pants getting hooked on the top of his pad? Causing it to drive up into his crotch area?
I'm with McHockey... it's probably the pants being pushed up by the pads, the top strap of the pads or the knee/thigh pads (attached to pad or separate... does he have any?)
What you need to do is get him into his gear on a carpet you don't care about and adjust the straps until you get the right fit.
Generally, the idea is that the pants should rest between the top of the pad and the thigh when down in butterfly. If the pad has a built in thigh protector (usually attached to the knee area) or he's wearing separate knee pads, the newer models of these are curved and generally meant to be fairly tight against the leg and slide up under the thigh pad of the pants. Here are some tips:
1) when in butterfly, he should be landing on the INNER knee pad, not the knee area on the body of the pad
2) in order for the above, the pad should be rotating on his leg
3) that means the straps need to be loose... try at least 2-3 adult fingers between strap and leg
4) the very top strap only serves the purpose of slightly bending the top of the pad to close the 5-hole. Don't have it any tighter than required to get it to bend toward the other leg in butterfly to close the 5-hole
5) the boot strap also needs to be loose enough to allow rotation of the pad. Depending his his hip flexibility, he may find butterfly easier with his toes pointed to the ice and heel up... strap needs to allow for this.
6) when you've got all the straps and adjustments right, his thigh should be protected by an "accordion" of pads. Thigh/knee pads closest to the leg, then pants, then top of goalie leg pads.
Then get on the ice and go into butterfly to make fine tuning there.