My seven year old bearded collie has a lump on his side at the bottom of his rib cage.
He has an appointment with vet on Wednesday but I can't sleep for worrying help.

It's probably nothing. Dogs get lumps and bumps all the time. I can totally sympathize about the worrying. My German Shepherd had a lump on her leg about 6 months ago. That was the longest week of my life waiting for the lab results to come back. (It was benign, but I have no idea was it was. This was the longest word I'd ever seen! Even my vet had trouble pronouncing it!) Good news is, even if it is cancerous it's can most likely be surgically removed. (Chemo may be necessary, but only in extreme cases.) Your vet will talk to you more about it, and let you know your options. Who knows, it may be something silly like an insect bite or an ingrown hair!

Fatty tumors like that are common, but, unlike your vet, I can't say that is what it is. You are correct about seeing the vet. Even if it is benign, you will want to keep a close eye on it, calling the vet if it ever changes.