How to get Canadian citizenship for son born abroad

I am a Canadian citizen by decent (my mother was born in Quebec) and I hold a Canadian passport. I live abroad and have only resided in Canada when I went to university. I fall short of the number of days required for the direct citizenship by decent for my son (only ~700 days of the 1095 days required). I have read that I can sponsor my son to get PR status and once that's approved he can apply for citizenship. How long do we have to be in Canada to get his PR? I've read one thing that says on his landing trip a border officer will sign confirmation of PR and because he's a minor their is no requirement that he has to physically stay in the county afterward (we can go back aboard). Is this correct? His landing trip can be a 3 week "vacation" and we go back home (abroad)? Then my next steps would be to apply for his citizenship while his PR is still valid or are their PR requirements that must be fulfilled before I can apply for citizenship?

Thanks in advance for any help or advice.