Seeking advice for my cat Oreo

Oreo has been sleeping all day and barely eats his dry food.. my sister said he still eats wet food. But for the past few days, Ive noticed a change in his behavior and he has his mouth slightly open all day. He's usually a very active cat but now he seems weak and even hesitates to jump over counters. Ive also noticed a slight swelling around his left jaw but im not sure what it is. There's no bad breath or any oozing. Im just worried about him 🥺