From the sounds of it, you were right to presume it was a hernia. It sounds as though it is either an abdominal hernia or a inguinal hernia. An inguinal hernia in guinea pigs, especially older males, is not all that uncommon.
My suggestion would be to at least let a veterinarian examine him. The hernia may be more operable than what it may seem.
As far as age, five years old is up in age for a guinea pig, but that doesn't completely take them out of the running for surgery. I have a 6 year old male, now going on 7, who I rescued, along with his "wife". I wanted to keep them together, without the risk of pregnancy, so I had him neutered. He did fine under the anesthesia, with no complications.
As with any hernia, if left untreated, it does run the risk of ruptering. Only a veterinarian, however, can really determine what the risk of it ruptering anytime soon are, as well as what the risks of the surgery would be. The extent of the surgery would depend on things like how deep the hernia goes on the inside, how many blood vessels are involved, and not only the anesthesia itself, but how long he'll be under anesthesia.
Now that it has begun to grow, I really would recommend that you, at the very least, let a vet examine him and give their professional opinion. Afterwards, they can determine an appropriate coure of action, whether it be to treat him, or to let him live out his days.
I hope this helps!
I am NOT, nor do I claim to be, a Veterinarian.
Always check with your vet for any symptoms your pet exhibits.
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