Yes, you should phone the police if you think it's necessary. However, be warned that that would be social suicide, and may not be best for his social life in the long run.
Boy's schools (or at least the one I went to) have an unspoken hierarchy, a kind of social ranking. If he's new to the school, he will already be at the bottom of the ranking. By getting the police involved, it shows weakness to the other boys, and so he'll find himself struggling to climb up this ranking, and therefore is more likely to become even more of a victim.
It seems silly to talk about a school in this way, but it really is true.
P.S whoever negged me has clearly not been to a boys school.