I have a 6 1/2 year old and he is still in a 5pt harness. I agree with the previous moms about it being your call about the video, but you're the mom.
I tell my son that its the law (Illinois is 8 AND 80 lbs). He has friends who are only in a booster and something was said once while they were in my suv and I said that he was safe in this seat and nobody every said anything about it again. He has friends who ride in the FRONT SEAT of their parents cars and he has seen that and asks why their parents "don't keep them safe" and I just say that the law is "age 8 and 80lbs" and right now, his 5pt harness will keep him the safest.
Educate your son as to WHY he is in the 5pt harness so even IF someone says something to him, he can respond with the fact that he will be safe in the car if something happens.
Kids are going to find all sorts of reasons to make fun of each other and as long as we, as parents, can help them deal with people saying things to possibly hurt their feelings.
I commend you for sticking to your guns with this. I would check out the youtube crash test dummy tests...I imagine that those would be so much less graphic. I have heard about the link that you posted and I cannot bring myself to watch it.
Best of luck!