If you are located in Britain, there is the possibility to boar date any single or bereaved boar at a good rescue under expert supervision. That maximises Justin's chance at finding a character compatible companion (the key to a stable boar bond) and minimises your risk of ending up with boars that won't get on.
Here is a list of recommended (and vetted) guinea pig rescues. The vast majority offer boar dating in some form.
https://maps.google.co.uk/maps/ms?msid=209994852075231951564.0004b8fd9391b4257d8eb&msa=0
Here is Suzy's boar dating blog from her rescue. She practises full (i.e. residential) boar dating, which is the safest, but also the most time consuming way to bond boars of all ages. Any resulting boar bond is however as stable as a sow bond. Not all rescues have the time to offer this specific service; some just offer "speed dating" with a youngster.
http://www.theguineapigforum.co.uk/showthread.php?t=59233
If you want to go down the neutering route, please be aware that it is crucial to find an experienced vet with a good track record in successful guinea pig neutering, as the risk of post op complications (and deaths) is sadly still far too common and can easily cost you a multiple of the operation cost, not to mention the heartbreak and sleepless nights.
You also have to factor in a full 6 weeks post op wait before your boar is 100% safe to go with girls. I have the surprise baby from a supposedly safe over 5 weeks post op boar (not one of mine), to prove that particular point!
Generally, cross gender pairings are the most stable, provided the boar has been initially accepted by his sow (not all will necessarily; so again dating at a rescue can help to make sure that he finds the lady of his dreams).
http://www.cavyspirit.com/neutering.htm
As to other boar options and boar pairings, you will find this link here helpful. We generally do not encourage trios with young boars; the fall out rate during the stroppy adolescent months is very high and most trios end up with separation.
http://www.susieandpigs.pwp.blueyonder.co.uk/optionsmenu.htm
Please give Justin a rag or cuddly toy to snuggle into in the meantime (if possible with his friend's fading scent on).