you don't need to provide an explanation for why he only works some months or on shift. cic doesn't care how many days he works. if there is a question that specifically asks for this information, that's where you would put it. if he's the applicant, id imagine the fact he doesn't work in the summer time would fall under the "activity" question in the application. again, it's not necessary to EXPLAIN. for example, on the question that specifically asks for all activity within a certain period of time, he would list the information for each job separately if they fall under different employers, and for the months he didn't work, he would list it as "unemployed" or "summer break". you do not need to provide a long story to explain it. as long as the REQUIRED information (which is written in the question or guide) is there, there isn't an issue.