I think the best thing is have a vet check and have him examined. It may not be a tumour. It does look rather red. Does he seem in pain at all? Ears down? Hunched? Difficulty walking at all? Or does he seem fine in himself.
If it helps, this was our first syrian sitting on his man bits when they were descended. They usually protrude when they're either warm or comfortable. The second photo they're not sticking out too much. On the first one you can see lower right on him that they are protruding and pink.
If it's like that permanently though you need to get the vet to have a look.
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