OMG, are you serious!
No, he will not be scarred.
I just spent 5 weeks in Puerto Rico where my family lives. During the weekdays, I left my 2 1/2 year old with my parents at THEIR house, where my daughter is not too familiar with the surroundings. I left her at my parents' house for 3 out of the 5 weeks, 5 straight days each time, only picking her up on the weekends. She cried as I was leaving the first time, but had stopped crying even BEFORE I had a chance to pull out of the driveway!
I tried to call every night, as you suggest, but many of those times she was so busy having fun with the Abuelos, and she did not want to talk on the phone. It just made things worse for her and for my parents for me to try to force her to talk to me when she didn't need to do it.
The calendar thing is a terrible idea. Even if he is "learning" the calendar at preschool, he doesn't understand what 5 minutes are, let alone 5 days. I tried the calendar with my daughter, and the calling every night. After getting better at this over the weeks that we were in Puerto Rico, I realized that my daughter needed time away from her Mami too. It was a "vacation" for her as well and I was intruding on her independence. Of course, my parents spoiled her! Don't take it personally if your son doesn't miss you at all and in fact, is an absolute angel with your parents while you are away :).
Please thank your lucky stars that your parents are willing to give you this amazing gift of staying in YOUR house for 5 days with your THREE kids! That is amazing. Accept this gift with an open heart and a clear mind and don't make it worse for them and for your son by complicating things. If you don't relax and allow them to be, your parents' sacrifice will be wasted. Just go and have a good time.