He will likely be asked purpose of visit, for how long, maybe where staying and possibly funds if an extended stay/ return travel arrangements
Generally an airline would expect someone to have a return ticket given without one they can challenge someone at home country checkin.
There is never any guarantee will be allowed to stay for any specific amount of time could as a visitor be a couple of weeks could be the default 6 months. no way to predict
With CBSA should always answer questions honestly, do not volunteer information to questions not asked. The fact that your family is completing a PR landing the following day may come up in the CBSA assessment as purpose of visit so be aware of that but only if asked.