have you thought about getting him a young boar if they dont get on then put them side by side in cand c runs but occasionally put them in a run together ( obvioulsy watch) but you never know he might get along with a young boar, I have lost a pig due to pining away when his brother and run mate died so would do everything I possibly could to cheer him up,
when Gingers brother Fred died there was no way I was going to lose Ginger same as tweety and sylvester, so I tried him with some of my other boys and he is now quite happily living with Bertie and dipstick, its 50/50 whether it worked but i took the chance and watched very carefully, that was over a year ago and I have never had any problems , obvioulsy I have to give the warning here to watch and wait and seperate at any sign of problems, but I would say a depressed guinea pig is not a nice thing to see, so good luck what ever you decide to do,