I'm not sure exactly how much credit he will receive, but I think 1.5 years for the 1.5 years full time, 1 year for the 2 years half time (but perhaps 16 months for this, because typically 30 hours is considered full time and 20 hours would be 2/3 of this amount) and between half a year and 1 year for the year that depended on classes taught. Any way you slice it, this probably amounts to 11 points on the FSW application for 2-3 years of experience. To get 13 points he would have to get credit for a full 4 years which seems unlikely here. Please note that to get credit for the different time periods, make sure the reference letter is very specific about the working hours during each different time period so CIC will not have any doubt or difficulty calculating it.
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Good luck.