well he had to way up the best course of action if he went there it could be a big mistake and make things worse if he phoned it could also make things worse.
he was unable to make a decision so he phoned someone else hoping they would have the answer.
I think you are spot-on with this. "Something is wrong at the farm." He doesn´t know what, he doesn´t know how bad it is, he is very much in doubt what his father wants from him, so he calls Julie for some advise. She obviously does not take it seriously (in her impaired and sleepy condition) and tells him to go back to bed; but Jeremy has heard his father´s voice and makes the decicion to call the police whether Ralph wants it or not. In a sense he makes a compromise by not calling 999, but the local police - it doesn´t seem as serious that way and Ralph will be less upset if some local coppers show up.
It all makes sense if you see it this way - and paints Jeremy as a rather insecure lad when it comes to decicion-making.