16 months ago i bought a rescued hamster,he was bitten up by his littermates,?

now i noticed his fur is thining out on his back,i change his litter twice a month,hes pretty clean,hardly scratches himself,hes got rather large testicles which he drags around.the fur thining goes from mid back to his testis.hes pretty active at night and is eating pooping and peeing normally.is age the reason for his fur loss?