I think if you're going to use a smaller needle for the ribbing, you don't need to subtract 10% of the sts. Use one or the other. And yeah, he probably does want that relaxed fit, but since the old sweater's cotton the new one isn't going to stretch out like that. Measure the waistband on the old sweater and use that for the ribbing. Also, maybe go up to size 4s for the ribbing, it'll still be smaller than the 6s you use for the sweater.
sue- knitting heretic