Help Me, Poorly Hamster

[Dpsmith]

Hey, My little Syrian Hammy is called Rex, and I love him very much I rescued him from a boring sad life. He has a wonderful life now of yummy food and lots of exciting things in his home he is friendly and has never bitten.
Recently he’s been quiet and I thought it’s because it’s getting colder and he’s getting ready for the winter but this morning I noticed a yellow scab looking thing under his chest/chin it’s got a little black scab in the yellow parts and this morning it was a little bloody. I’ve searched the internet world for help & he hates me looking at it. I tried to investigate to take a picture to try explain but it’s impossible. Some info he is almost 3 years old

Please can somebody help?

Dan x