My Cat's Been Stung On His Foot -bee/wasp It's Swollen Badly. How Can I Treat This Or Should I Leave It Heal Itself? He's Still Putting Pressure On It But It Looks Painful.

My cat always plays with bees and wasps. He's been stung many times before but I've never noticed this type of reaction to it. The swelling looks bad but he's still running around and eating well. It happened about four hours ago, but the swelling has happened gradually.