Introduction tips?

My chin Apollo lives alone. He has a ton of toys, human interaction daily and a fleece cuddly toy but I think he is depressed. He sits in a corner of his cage most of the time, only moving to eat, poop, drink, pee and be played with. I don't think it is a health problem because he has been like this a while and has not gotten worse it is just that I have tried several different things (fleece toy, new toys, slightly longer play times) but nothing is working. I have talked to his breeder who previously said he didn't need a partner but now she agrees I should get him a friend. Apollo cost £70 but this new chin she is offering used to live with Apollo, is about the same age as him and is only £40. She has also offered to lend me a cage for introductions. She would've done it but sge works away so she won't have time. I am now researching it but I am getting a lot of mixed responses. I will only do this once I am 100% confident and yes I know they could not get along and I am prepared to buy another cage. Please could you give me some tips on introduction, like everything that needs to be done? Thank you (sorry if this is in the wrong section)