I have kids who have gone through middle school and so I know a bit about this.
What I found surprising is that hijinks between kids who are considered friends (or at least even in the same friend circle, or larger social circle) are much more common that you'd think. Borderline bullying ... this word bullying I find very confusing. That's why kids (especially boys I find) don't know if it's bullying or just pranks between guys in the same circle ... in any case, it's not very nice.
I had one kid who was hyper and well, out of control at times, basically attack my son. They were friends! Or had been the year before. They were in the same social group too. I didn't get it. My son was coming home having been pelted with stuff (fruit?) that falls out of trees (not sure what they were) and bruised from lunch at school. He was upset about it, his friends didn't know whether to stand up to the kid or not, as he was supposedly a friend. It was just weird - teenage boy stuff. My husband said "ya well ... they did that at our age too ... ". The teachers were vague about it also. The kid was nice otherwise to my kid.
Was that bullying or fun gone too far? Because it had started in fun apparently. My kid was not involved in it though - his friends were. I have seen that at our house too though. Between my boys ... or when we have friends over. Without an adult there supervising closely, sometimes things escalate and get out of hand very quickly. I'm not talking fighting - I'm talking silliness that gets out of hand. Very quickly. To the point where it's upsetting - more embarrassing to the boy it's happening to.
So I would say this white out thing might fall into that category. As opposed to say, a kid being targeted day in, day out - being stalked and bothered by someone trying to oppress a kid. I get that both instances are truly horrible for kids - and I'm not sure where you draw the line. But I too find it hard to know when to intervene.
The pelting my kid incident - I did call in. Because we had a history with this kid and I was tired of it. However I handled it like this - I said there was a kid, in my son's social circle, who was getting a bit out of control, and were they aware of it? because my kid was coming home bruised. Where was the supervision? And I left it at that. I sort of called up to make them aware of it. I said things had escalated at elementary school and I wanted to be sure they didn't at junior high where they were bigger, etc.
It's tough. I'd keep encouraging him to deal with it himself as much as possible. I too saw a great video on Facebook not long ago (mentioned below I think) where bullying was demonstrated in a school as being all about power - and if you react, the bully keeps on bullying. They want that reaction. If they don't get it, they have no reason to keep doing it. Then they feel stupid. So our kids tried to not react. Blow it off. Like "Whatever... ok then". Tried to make the bully kid look like a loser for bugging them - like they had nothing better to do, then they'd go off with their friends. It doesn't always work - but there's that fine line with tattling and looking like you don't care.
If you can report the kid as someone the teachers should watch out for so that THEY can oversee and catch it happening (cuz likely happening to other kids too) then all the better. Good luck.