Thank you for taking this bird in!
I would spend a lot of ‘passive time’ around him, so Reading for example like Tanya Said, singing if you like that, do your Own stuff around him and talk to him(try not to look directly at him, he might percieve you as a predator).
Keep enough distance however, sit at a distance where he doesn’t show signs of fear.
Every time you enter the room he is in, or walk by his cage, give him a treat. You can give them to him directly, but if he does not take treats from hand, you could add an extra treat bowl to drop the treat in.
I would pay a lot of attention to his body language, every time he shows fear, back off. When my lovie Fëanor came home and I accidentily scared him, I would back off, look at the ground and usually lower my whole body a bit. He quickly learned that if he was scared, I would back off and not force myself on him. Besides, I(the scary thing) could be removed if I became too scary. That Made him more confident. So any time as he showed a sign of fear because of me, I would remove myself.
I would keep him flighted, his wings play an extra import roll for him because of his missing toes. He uses them to keep balance. One of my lovies has a foot that does not work well, and he uses his wings to regain balance all the time. Clipping him Will also remove his ability to flee from the danger(us scary humans!), he Will probably feel Powerless. You can compared that to a human who is paralized by fear I think.
I believe it is possible for him to have a lovebird friend in the future, but it always a gamble if two birds Will get along. In your case, I think your best bet would be to look for a lovie that is a bit mellow and easy going.
I used these things to build trust with my lovie Fëanor. Two weeks after he came home, he was napping on the tip of my finger. Bonding is a lot of fun, good luck! Keep us updated of your progress!