Sounds like the pattern needs a bulky yarn and you have a worsted, or else you knit tight. You could try an 11 and see how that goes, but it may be too thin for your yarn. You don't want the sweater the same size as his chest measurememt though, men like sweaters that are 2-4" larger than they are. So you actually want something about 40" around. Multiply your gauge times 40 and see which larger size is closest to that measurement (might be medium, might be large) and use that. But for the lengths, where it says to knit to X inches, use the measurement given for the smaller size.
sue- knitting heretic