He had diarrhoea today when I walked him and now after I got home a few minutes ago he's shivering, panting a little bit and off his food. Will I leave it till tomorrow to bring him to the vet or will I call them now?

I usually give it 24 hours if it's just a bit of the runs., sometimes the shaking can be because they don't feel well.

Is there any chance he could have eaten anything toxic?

For a normal upset stomach I would normally leave it overnight but it's always a difficult judgement call. I could easily have lost my dog due to leaving it over night. I had no idea what was wrong at the time but it turned out she had picked up a stick awkwardly and one end had pierced her throat. They had to operate. With the panting and shivering in addition to the diarrhoea I would call a vet.

How is your dog OP?:

It's very frustrating when people post requests for advice then disappear. OP what happened?