Can you do a quick search for a paralysis tick? TBH alarm bells ring ANYTIME a labrador gets lethargic. I would recommend going straight to your regular vet and waiting for an appointment. They will fit you in if they see him in this state. Is he able to wag his tail? If not, this is something that needs seeing this morning. I don't want to scare you but I don't feel this should be ignored.

I've worked as a vet nurse for over 10 years, and for the last 8 months I've been working in emergency at a Vet Specialist Centre.