Its great you are looking to get him a friend, he will definitely be happier!
For a bond to work, two piggies have to have character compatibility. The only way to guarantee they have compatibility is to go dating at a rescue centre. The centre will try your piggy with other available piggies and find a match. When successful you will bring them both home to continue to live together.
If you buy a piggy on spec from a pet shop for example, then as you won’t know the character of the new piggy, it may work out perfectly fine, but you would need to have a plan in case it did fail. Youngsters are often accepted by older piggies but as the youngster hits his teens, there might be a chance he could challenge for dominance and there could be issues if they both have dominant characters. If a bond fails, then they would need to live in separate but side by side cages for companionship though the bars only.
The cage for two boars to live together needs to be large - 180x60cm is the recommended size - as boars are territorial and need more space. Too small of a cage can cause tensions and problems.
Your boy is still just about in his teens (until around 15 months of age). It can be harder to bond teenage piggies due to the hormones. Its often recommended to neuter, have the sis week post op wait to become infertile, and then look for a compatible sow.
If you are to carry out a bonding yourself at home, then it needs to be done on neutral territory (somewhere your boy doesn’t see as his teritory). The guide below explains the bonding procedure, as well as other guides for further information.
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
A Comprehensive Guide to Guinea Pig Boars
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
Dominance Behaviours In Guinea Pigs
A Closer Look At Pairs (Boars - Sows - Mixed)
Neutered / De-sexed Boars And Neutering Operations: Myths, Facts and Post-op Care
pls help me !