It’s not hard to bond boars - no different to bonding two other piggies - the essential part is that two piggies must be character compatible.
Boars can be bonded at any time and at age. Being alone all this time is not necessarily a barrier to him having a friend and getting him a friend is the very best thing for him.
If he learnt social skills as a youngster then hopefully he will still remember how to interact. The issue can be if he was separated and never saw another piggy again so didn’t learn. Even in that case, it doesn’t mean a bonding will fail. Even if it did, he will still benefit from having a piggy live next to him in a separate cage.
The safest way to find a new friend for him
Is with the help of a rescue centre if one is nearby to you.
If you are buying a new piggy then you need to have a plan for if the bonding to fails so they can live side by side (cage size information below but if the bonding fails a 12 sq ft cage divided in half is not big enough for two piggies to live side by side).
Age and physical size are not really relevant. What does matter is that the two of them are character compatible - they have to like each other and be able to form a functioning hierarchy with one being dominant and one being submissive. If they both want to be dominant then they will fight and the bond will fail.
It’s can be easier to bond with a younger piggy, a baby, as that baby will be much less likely to challenge for dominance during the bonding. However bonding with a baby can still fail.
If you do get a baby then the cage must not be divided at all - they need to be bonded in neutral territory on the same day that you bring the baby home. A baby is too young to be alone.
You put both piggies in a neutral territory pen (not in the normal cage your current piggy lives in). You then leave them in the bonding pen for several hours to get through the early stages of forming a relationship. After a few hours, if the bonding has gone well, you move them to a thoroughly cleaned out and also neutral cage and then leave them together permanently. No divider is ever needed in the cage. It will then take around two weeks for them to fully form their relationship.
If you get a piggy that is not a baby, then you can put them in a divided cage for a time so they get to know each other through the bars - that can be for a few days prior to bonding day. However you still then need to put them into different neutral territory pen for bonding. Same process as above - leave them in the pen for several hours while you see if they like each other. If they do then you need to clean out the cage they are to life in, remove the divider and make it also neutral. You can then move the piggies into the cage and leave them together from them on.
What you cannot do with boars is put them together and separate repeatedly. They need to be put together and left together permanently or if the bonding fails, then separated permanently.
If the bonding with any new pig fails, then the two piggies will need to live separated but side by side and not have physical contact without bars between them. This will enable interaction with each other so neither gets lonely but means they don’t have to share a territory.
In this case, the cage you have is not big enough to have a divider in it. A 12 sq ft divided in two only gives each side 6 sq ft but each single piggy needs a minimum of 8 square feet. So if the bonding fails, then you will need to make each side of the cage bigger.
The guides below explain bonding more fully
A Comprehensive Guide to Guinea Pig Boars
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
Moody Guinea Pigs: Depression, Bullying, Aggression, Stress, Fear and Antisocial Behaviour