The covering letter is simply a formal application for the position, with the CV attached.
However it can be useful to emphasise in the covering letter any attributes/skills/qualities of the applicant that are particularly relevant to the job in question.
If any of the mandatory job requirements are not captured within the CV – consider amending the CV accordingly, otherwise state that you meet these requirements within the covering letter. If he has no qualifications/job experience relevant to the position, then stressing his hobbies/interest in these work activities can be included within the letter.
For example, it might be a requirement for this job that the applicant is a strong swimmer.
Once you have produced the cover letter, get friends/family to critique it; and check it carefully for spelling/grammar mistakes (such as an aquatic, not a aquatic).