There is no way to guarantee his entry to Canada unfortunately, but you likely will not run into issues since he is American.
As long as you are applying outland, it does not matter if you start the process before or after you enter Canada, nor will it have an technically have any impact on his likelihood of successful entry, since filing the PR app does not grant him any kind of status. He will be entering as a visitor either way.
My personal recommendation would be to file the app as soon as you are ready, and if you'd like to come to Canada while it's processing, don't do anything crazy (like sell your house in the US etc) until your husband has successfully entered Canada as a visitor.
Thanks so much for your advice! My husband has successfully entered Canada multiple times as a visitor before (most recent was July - for my sister’s wedding). The thing is, if we make this big life change, we WILL have to sell our house, and pack up all our belongings in order to move to Nova Scotia. This will all have to be planned well in advance so we can make this move. We are kind of “done” with our current house though and ready to move on regardless, but we will need to sell it and move all our furnishings, cat, vehicles, etc., to start our new life in Nova Scotia.