I can't give much advice since I'm not a veterinarian

I'm sorry.
I'd say if you JUST (today) found this lump and your dog is acting NORMAL (eating, drinking, going to the bathroom and running around) - to wait until the vet is open.
However - if your dog is acting ANY different than he usually does - laying down more often, having problems breathing, eating, drinking, problems going to the bathroom take him IMMEDIATELY to an emergency vet.