- And got the T-shirt
If you can find a spayed sow, or if you want to neuter your pig, a male-female pairing almost always works. Just don't get an unspayed female, as pregnancy and delivery are very hard on guinea pig sows, and the death rate is high for both sow and pups.
Please don't go buy a pig to put with him. There are thousands of unwanted guinea pigs in rescues and shelters that need good homes.
However, you'll need a large cage -- two males need 10 square feet at a minimum, and more is better. And if you get another male, you WILL have some squabbling -- it just comes with the territory. But it doesn't mean they can't live together mostly peacefully, and enjoy each other's company.
- Supporter in 2019
As bpatters mentioned, you'll need the largest possible cage you can manage for two males. DO quarantine any newcomer unless they have come from a reputable rescue (which is HIGHLY recommended), and DO introduce the two pigs in neutral territory. See also: