He probably has a bacterial infection. Pepto could help SOME but yes I'd take him to the vet, not necessarily the ER one but your regular one, tomorrow (as long as you can get in tomorrow). Fast him if you're not already. If the diarrhea gets bloody then you may want to take him in sooner (to the ER) If it's a bacteria like E. coli or salmonella, he'll need an antibiotic.