Your vet needs to give your hamster a full exam. It's not a good idea to start medical treat on him. As you said you don't know what the red lump is, therefore you can't properly treat it. The vet may need to aspirate the lump for a proper diagnosis.
My Russian dwarf hamster has a red lump on his back left foot, he is around 2 years old. What can I do to treat it?
A pet store may know whether this is a common issue with this type of pet. You could call for advice.
The best bet is to take him to the vet, though.
The vet may need to aspirate the lump for a proper diagnosis.