If he's left handed, and assuming no form/release/nock fit problems, a "right" group would indicate stiff, wouldn't it??? Don't cut those arrows yet!
frozen bow: Try this... if you have some heavy field points, try bumping up your point weight and/or drawing just a little farther than you normally do. If the arrows come more in towards center, you're probably a bit overspined with those 250's. Also, you could try reducing point weight and/or shortening your draw a little, and see if it gets worse or better... Do a little field expedient testing before cutting or changing arrows!