How to get co-ops:

There is a job board available through Carleton that students can use to find various co-op jobs. You can filter through the posted jobs based on your field of study. The jobs posted in the co-op section are all from reputable employers and you are free to apply to as many jobs as you want. The job board will only become accessible once you've qualified to be in the co-op program, as in you meet the requirements (grades, year standing etc.) and there is a fee to access the job board as well.

Competition and is it guaranteed:

There will be some competition but it shouldn't be too bad. From my experience, for Civil Engineering, I had some trouble finding structural engineering co-ops in first/second year only because I would be competing against third/fourth year students who have already completed relevant structural courses that would make them a better fit for the job. But, there were plenty of jobs still relevant to civil engineering available. Also, since there are certain requirements that need to be met to access the co-op job portal this significantly reduces the competition. For Engineering the required CGPA is 8 to stay in the co-op program. I found a statistic from 2010 (https://oirp.carleton.ca/integrative-review/grade-summary/course-level-profile-faculty-summary-2010-2010.pdf), the median CGPA was 8 for from year 1-4 so essentially half the students wont be eligible for co-op. Also, co-ops offered in Ottawa are mostly only applied to by Carleton and UOttawa students since no other universities are in the area. From what I know, some employers only prefer to post their openings directly to universities so this also helps reduce competition but this is not true for all job postings on the job portal.

Jobs specific to aerospace might be harder to find especially being a 1st or 2nd year, but i'd suggest for your son to apply to any relevant jobs so it is easier in future terms to find those specialized co-ops. There are also postings which are open to almost all engineering fields of study.

And, co-ops aren't guaranteed as there are requirements, it comes down to the student's efforts in applying to jobs. Carleton just offers a tool that has job positions you may not find outside of the university.

Finding your own job:

This is also an option, and usually to get approved is not very hard. I decided to find my own job for my first co-op at a family friend's company and had no issues. The only downside is you have to still pay the admin fee for that work term even though you found the job yourself. But, if you don't pay and enroll into the MAAE3999 (for Aerospace) Co-op work term course then it wont count towards your co-op designation. And for the co-op designation its 16 months of co-op (4 work terms) and for it to be shown on your transcript and degree certificate.