Help with hamster please!!!?

I have a male chinese dwarf hamster. And i don't no whats wroung with him. Today i noticed that all the area under neath his tail is red raw like he had been biting it or something. I first noticed a small wound near his tail a week ago. And thought it was fine but now its got worse. I don't know wether its serious or not. Should I take him to the vets?