Unfortunately I don't have too many tips, but did just want to throw in some additional info for you - while United is primarily out of Term 1 at ORD, the gates in this terminal are divided into two different areas (B gates and C gates, I think) that are connected by a long-ish underground tunnel. If the arrival gate is at the far end of one side and the departure gate is on the far side of the other one, it could potentially take 20 minutes or so to walk from one to the other, if he's moving a little slowly. On the other hand, if both gates are on the B-side or C-side, it probably wouldn't be far at all and 40 minutes would be plenty of time, given no arrival delays. In either case, asking for help may be a good idea!