It is not the case that he is too old for a same sex companion. Piggies need companions and can be bonded at any age. However, any bond comes down to character compatibility - the two piggies have to like each other to be able to form a bond and functioning hierarchy. The best way to find a new companion is to take a piggy dating at a rescue centre so the piggy can choose his own new friend. That way you can be sure they are compatible and will get on.
Bringing home another piggy that you have chosen yourself, does risk that they won't be compatible and therefore won't bond
Slipper is a teenager now, so bonding him can be slightly trickier due to the hormones, but he really does need a friend.
As your piggy is a boar, you can only bond him with one other male piggy - you an only successfully keep two boars together. If you were able to neuter him and have the six week wait to become infertile, then he can be bonded with a sow (or as many sows as your cage space allowed).
With that said, its not fair to keep a piggy alone for their whole lives (anything up to 8 years) as they are highly social creatures. If you do bring home another piggy and they dont get on, then they will have to live in separate cages but the cages must be side by side so they can interact through the bars. This is still preferable to being totally alone. Living in side by side cages means they can still talk to each other (they interact via, sound, smell and sight) so neither piggy will be lonely.
For two male Piggies to live together in the same cage, the cage needs to be 180cm x 60cm.
You must carry out a bonding in a particular way - on neutral territory. Please dont just put a new piggy directly into the cage of the existing piggy as it will cause fights before you even start!
I will add guides below which detail boar behaviour, how to bond two piggies as well as other behaviour guides
A Comprehensive Guide to Guinea Pig Boars
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Moody guinea pigs: Depression, Bullying, Aggression, Stress, Fear and Antisocial Behaviour
Neutered / De-sexed Boars And Neutering Operations: Myths, Facts and Post-op Care
A Closer Look At Pairs (Boars - Sows - Mixed)