Having a PR means freedom to work. However, while you're waiting the 12 months to get PR, you may not be allowed to work unless he has another work permit (you've indicated he does, for co-op. He may also be eligible for PGWP). Depending on when you apply, when he graduates, and processing times, you may not need the OWP.
However, if he will graduate soon, and will not receive a PGWP, you will want to think about his status in Canada. A lot of people like to rely on 'implied status' in inland applications - that's the status that you get while you wait for an application to process. However, the PR application itself does not confer applied status, but the OWP app does. So, applying for an OWP if he is soon to lose status means he can continue to remain in Canada on implied status until the OWP app is processed, and provided it is given to him, he can work.
There's some complicated things about implied status and working permits as well, but I don't know it enough.