I’m glad you’re looking to get him a new friend. It is so important that they aren’t by themselves. They do get lonely (won’t die from loneliness), but guinea pigs aren’t wired be by themselves.
Companionship
the most important thing about getting a new friend is character compatibility, not age. You can keep two piggies who aren’t the same age together but they must like each other, want to be together and be able to form a hierarchy.
the safest way to get a new friend for your piggy is to take him dating at a rescue centre. That way, he can choose his own new friend so you can be sure they will have compatibility.
If you get a piggy from a pet shop then you won’t know if they will be compatible and there is then a risk they won’t like each other, they won’t bond and could fight. if that was to happen, they would need to live in separate but side by side cages for interaction through the bars.
As you have a male piggy, you can only put him with one other, character compatible, boar. You can’t keep more than two boars together otherwise there will be fights. Your other option is to have him neutered, have his six week post neutering wait for him to become infertile, and then he can be bonded with a sow (again, via dating at a rescue centre is best). You can keep one neutered boar to any number of sows - space allowing of course. You need to make sure your cage is big enough for any number of piggies.
Neutered / De-sexed Boars And Neutering Operations: Myths, Facts and Post-op Care
Two boars need a cage of 150cm x 60cm as a minimum, but 180x60cm is recommended. Two boars need lots of space to have enough territory. A neutered boar and one sow need a cage of a minimum of 120cm x 60cm but 150x60cm is recommended.
Cage Size Guide
there is a procedure to having guinea pigs meet and if you Do go to a rescue centre, they can often do it for you. If you were to have to do a bonding yourself, then this guide explains what to do. The introduction is a one time thing - you put them together Snd see it through to conclusion and if successful, they then live together from then on. It must always Be done on neutral territory, never put a new piggy directly into the cage of an existing piggy as that will cause a fight.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Please do read all the green links to guides I have added in. They contain more information than we can relay in a post.
these are some more useful guides about guinea pig boar pairs and their behaviour, as well as one about keeping a neutered boar/sow Pair.
A Comprehensive Guide to Guinea Pig Boars
Dominance Behaviours In Guinea Pigs
A Closer Look At Pairs (Boars - Sows - Mixed)