2.5 years = 912.5 or 913 days that you would be absent from Canada. So, as for yourself, if you do a quick landing and then return in 2.5 years and stay, you will be OK. However, keep in mind that you must be in Canada for at least 730 days within the 5-year period starting with your landing date to retain your PR. If you, after moving in 2.5 years leave again or come in and out of Canada, you will have to ensure that you meet the 730 day requirement to avoid possibly losing your PR status.
The same applies to your wife and son. They must meet this requirement also. They do not lose their PR status unless they don’t meet the Residency Obligation (RO) requirement: 2-years within the 5 years beginning on your landing date. (aka 730 days in that 5 year period)
If your wife and son do not join you until after 4+ years they will NOT meet the RO – that 730 days that they must be in Canada to retain their PR. As someone mentioned earlier in a post, they could try to come in and do an appeal to stay since you would then turn around and re-sponsor them. I don’t know if that would work. There is a thing that you can renounce your PR, and then you could re-sponsor them, but I don’t know if it’s better to do that or wait until their PR would be revoked. Not sure what is better between those options. In any case, if they lose their PR, which, unless they win an appeal, they would as they didn’t meet their RO, then you could re-sponsor your wife, and, if your son is still of dependent age, you could sponsor him as well. Currently the dependent age is 22. It was raised by to 22 by Trudeau after the previous PM, Harper had lowered it to 19. So that age could change. If your son is nowhere near those ages he should be fine to re-sponsor.
See this link on PR Status:
https://www.canada.ca/en/immigration-refugees-citizenship/services/new-immigrants/pr-card/understand-pr-status.html