Hello and welcome. My name is ***** ***** I'm a biologist who has over 40 years of experience raising guinea pigs and rabbits. I'm sorry to hear of this problem. There area several possible causes of this symptom.
First, check to see if he may have injured his underside by scraping it on something. If he has, you can treat that by cleaning the area with Betadine (sold in first aid departments of pharmacies and discount stores). Follow that with a light application of plain Neosporin. The Neosporin would be repeated each day until the area is healed.
Nipples can become infected in both sexes. Leakage of fluid or blood is common with these infections. Treatment would be with a safe prescription antibiotic, such a s Baytril.
While injury or infection could certainly be responsible, the most common cause of blood from the nipples is a tumor. Tumors can be benign or cancerous. Guinea pigs are very sensitive to anesthesia, which makes them poor surgical candidates, but an experienced guinea pig surgeon can remove tumors safely. At age 6, your guinea pig is a senior, so if he has a tumor, you'll have to discuss the risks and benefits of removing it from an elderly guinea pig.
The best thing to do is schedule a vet visit. This link will take you to a directory of guinea pig vets:
If you have more questions, let me know in a REPLY. I hope your guinea pig will be just fine.
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