My son was beaten up by an older boy last year. He was 13 and the other boy 16, nearly 17.
Ds1 did not want to call the police as he did not want to be seen as a grass. We had a long talk about how this boy wouldget away with it and might go on to do it to someone else. In the end he agreed to talk to the police as he knew he had the power to save someone else from getting the kicking he got.
Police were amazing. It took a while for them to get to us as the nearest station is 10 miles away and the priority for the 3 officers on duty was the drunks in the 2 market towns they cover. But when thw arrived they were brilliant: really listened to ds, asked him questions and told him clearly what the next steps would be.
They knew the boy's girlfriend so soon found him, and I gather the talking to they gave him scared the shit out of him. They made it quite clear that he was very nearly an adult and that adults who assault kids are not treated with much sympathy. They told him to stay away from ds.
Ds was approached by the boy about a month later. He had the dog with him so couldn't run away, but the boy just said sorry and asked if they were cool. Ds squeaked out that yeah, they were cool, and that was that.
OP, the police are used to dealing with reluctant, frightened teenagers. If you've told them he's reticent they should treat him carefully - the police are on a huge public image drive with kids at the moment so they'll be especially careful to keep his trust.
Chubbychops, you have given some of the worst advice I have ever heard in the 13 years I've been on MN. Your anecdotal experience sounds unfortunate but to persuade others to put up with violence and bullying is spineless, ignorant and downright dangerous.