Have you looked at the Barmy4Boars website:
http://www.susieandpigs.pwp.blueyonder.co.uk/optionsmenu.htm
You are in the same position I was in about 15 months ago. We had 2 brothers, Sammy and Snug. Sadly, Snug died and Sammy was left alone. Sam was then 4 years old, too old to risk neutering, so the options were to pair him either with a baby boar or with an adult.
Pairing with a baby is often recommended as the easier option, as the baby will accept the older boar as 'boss'. But the downside is that when the younger boar reaches maturity, he may launch a challenge for boss spot (!) This can result in conflict a few months down the line, when you are perhaps no longer expecting it.
Bonding adult boars can take much longer than introducing a baby. When we introduced Sam and Roo, it took us three weeks before we left them in a cage together unsupervised. But the good news is that, with bonding adults, you get all the conflict in the early stages when you are alert to it and watching, and once this stage has been successfully passed, you can get a really strong friendship.
You don't say how old your little chap is...if he's still young, then a third option would be to have him neutered and get him a girlfriend. There is always a risk in this, and it depends on how experienced your vet is. Even then, not all boars/sows take to each other, but the advantage is that a rescue will let you take your neutered boar along to meet a sow before you adopt. You can not do this when introducing adult boars, as they will take time to get to know each other. Also, many rescues refuse to let boars be adopted to live with other boars as the staff believe it can not be done and they will fight.
Hope that is some help! If you decide to go for an adult friend, I can give you more info on how we did it with Sam and Roo.