You are correct in believing that your brother will need to go through U.S. Preclearance in YUL. There is a direct path from intl. arrivals that does not require to go through Canadian immigration first (just a cursory check) and his checked luggage will be forwarded automatically.
If the inbound flight is on time, 70min is ample time to make the connection. If the connection is missed, he will be reprotected on the next available ORD flight (and there are several on AC and UA each day).