My cat has a sore on his neck. Is it serious? Should I see a vet?

Thanks for using Petco Pet Education Center, formerly Petcoach! I'm sorry to hear Ringo has a wound on his neck. Based on your picture, it does look like he might have a skin infection of some kind, even possibly from a ruptured abscess. Does he go outside? I would recommend having him evaluated by his regular veterinarian to have area clipped, cleaned, and assessed to determine what may be going on, and what treatment (antibiotics, anti-inflammatories, or possible surgical repair) may be needed to resolve it. If he goes outside and the wound may have come from an unknown animal, I would also recommend his rabies vaccine be boostered as well. Good luck!