You might try another lovebird in its own cage. At 9 years old, and given the rest have passed away, the remaining lovebird probably would not work out well with a young bird. If you plan to keep having pet birds, then I would go with a bird in another cage. If you are only thinking the older bird needs a companion, I would probably leave things as they are, get the bird a mirror and make sure he has plenty of toys. Otherwise you will get in a cycle of replacing mates. If this bird is from the same breeder as the others, chances are his life span will be similar so he may not have a lot of years left. Lovebirds can live into their 20’s, but a lot is determined by genetics as well as diet, so you have to look at the fact that the other 3 only lived around 9 years. And he might be a lot older if you didn’t know their ages when they were given to you.
Thank you for asking Lafeber,