Hair pulling

[Worrywobble]

For a couple of months, our nursery has been concerned with my 2.5 year olds hair pulling. He only does this to other children so as he's an only child, we've struggled doing our bit to drive the point home to him that he must not do this.
I can hear my DH upstairs right now explaining that we do not pull hair but DS is just laughing and saying ok.

I have said to nursery that being quite firm and explaining clearly that he mustn't do has worked with other unwanted behaviours but this doesn't seem to be working for them.

I was handed an ABC chart today and explained they will be using this to monitor him. I'm not particularly optimistic as it seems to come completely out of nowhere.

Should I be worried?

I don't think you should be worried. It's very common at that age. I'm surprised they've introduced an ABC chart to be honest. Maybe they are looking for patterns to see if there is anything that triggers the hair pulling.
For me it would just be ensuring that you & the nursery are using a consistent method to ensure he knows it's wrong. It will be a phase. Then there will be another phase.