No. He cannot work for the subsidiary until the petition is approved and he is admitted in L-1 status. As a Canadian he needs no visa stamp after approval, and premium processing can shorten the wait, but the season does not change the rule.
Approval, then admission, then work
The legal sequence is approval and L-1A admission before U.S. work, but a Canadian citizen can use either of two petition procedures. The employer may file Form I-129 with USCIS in advance and the employee may present the approval notice when applying for admission, or the signed individual petition may be filed with his application for admission at an eligible Class A land port or U.S.
preclearance or pre-flight station in Canada. Only after the petition is approved and he is admitted in L-1A status may he start managing the U.S. crews.
The filing can occur as soon as the continuous year abroad is complete and the acquisition documents exist, so an owner planning an April start should prepare in the previous autumn or winter; premium processing is available for an advance USCIS filing if the calendar is tight. Before admission, the owner may visit the subsidiary as a business visitor to attend meetings, inspect equipment or negotiate, but not to run operations, supervise crews or bid on contracts as the U.S. manager.
Doing so is unauthorized employment and can damage the petition. If the first season begins without him, put the subsidiary's existing manager in charge on paper and in fact until admission, and record that arrangement in the file.