I find it difficult to contemplate that a 13 year old could outrun a rottweiler, but that's a matter of nuance in this case.
You should report this to the police. You should also go to the site and check for any cctv cameras yourself. This would include checking sites where the man and his dog potentially entered or left the area (perhaps even car parks).
If any cameras are controlled by a public authority you can make a freedom of information request for anything recorded. If this is refused on the basis that it would identify others, then ask again, pointing out that it is the authority's responsiblity to provide the information, and protect the identity of others (ie they have to provide what is requested, and blur out faces if necessary).
If you can get cctv material then the police will become more interested (even if id obscured), because they can get the unobscured video, and perhaps identify the person. They are overwhealmed and don't have enough resources to be chasing relatively minor crimes where the culprits are unidentified and there is no other evidence.