You cant just get him a friend and expect them to get on, they will fight and you could could end up with a dead guinea pig. Please do not go to a breeder or a pet shop to get another one. Boars have to pick their own friends, hence the going to a rescue and letting them pick each other.
What pellets are you feeding? How much hay and veg does he get? How much in grams are you trying to get him to eat daily?
Where are you located, a town name will do or even a county if you dont want people knowing where you live, PM me if you want with the info, I am only trying to help you here I have said several times that people will help you out. Please never rule out rescuing as its the only option when getting a lone boar a friend. Boars sometimes have to meet several other boars to find one they like. If you let me know where you are I can tell you of rescues, see if anyone local can help with transport and let you know of the rescue names etc etc
Please do not rule out rescue with such a blanket statement.
Your cage needs to be 4ftx 2ft or larger otherwise a rescue will not let you have another boar. Smaller than 4ft x 2ft is too small for a single guinea pig anyway so id hope you have atleast that size. You say you are a member of the guinea pig forum so you should know all of this stuff anyway
Please never get a pet without thorough research beforehand as you can end up in awkward situations :-=)