How long has it been since you rescued him from the street ? Please post some pictures of him as soon as you can so that we can see him from all sides. Need to be able to see entire body, but as close as possible at the same time.
He needs to eat in the morning and in the evening with a little to nibble on through the day. What he likes is going to depend on what he was accustomed to eating. So start off with some cooked brown rice and add some mixed vegetables, such as peas, carrots, sweet corn, you could put in some whole grain bread until you can get some whole grains, add some pieces of apple without seeds or peel. If you have some leafy green vegetables give him some as well, but not head lettuce. and a seed mix for dinner and remove it after he goes to sleep.
I give mine a cooked mix of white beans ( all beans must be well cooked to be safe ), brown rice, whole grain pasta and the vegetable type pasta, mixed vegetables, whole grains, apple, squash both yellow and green, raisins, dried pineapple,and or other fruit. This is breakfast and I give fresh fruit and vegetable ( raw ) to nibble on and then a seed mix for dinner.
This link
viewtopic.php?f=8&t=12521 has a list of foods that are safe to give them as well as a pot of other things that you need to know about that are safe or toxic for him. The list begins on page two. Please be sure to read it.
Important !!! If you have non stick cook ware do not use it as if it get too hot it gives of a fume that will kill this bird in seconds, and be sure that his cage is not place in the kitchen.
We are going to need to know what size his cage is as well as where you have it placed, again pictures help a lot. You will need a supply of bird safe toys, pieces of crumpled paper works in a pinch as they usually love to shred paper. He needs to be near a window so that he can get sunlight, especially at dawn and dusk.
I understand that you are not yet ready for this part of it, but this bird will need to have at least one hour of one on one time with you and four hours of free time out of its cage every day. You have a lot of work ahead of you to make it safe for him to come out of his cage. You need to make sure that there are no electrical wires or cords that the bird will be able to get to and chew on, you need to make sure that there is no place that the bird can get into and get trapped or that you can't get to quickly in case that you need to save him. Some house plants are toxic to birds and you will find a list of them on the link that I have already provided to you. So you may have to move them to where he can't get to them. Other animals can be a major problem, so if you have other animals don't try to introduce your bird to them or allow them to interact with each other, so if you do have other animals in the house we need to know what they are.
OK ! Lets move on to you and the bird and how to start building a good relationship. You will need to set aside some time each day about 15 minutes at least twice a day more if possible. I can not tell you how fast or slow this will progress as it depends on too many factors that neither of us are able to answer.
So to begin your session, enter the room that has the bird in the cage and stop for a minute and look at the bird and notice what it is or is not doing. don't look straight on but sort of watch from the corners of your eyes or watch by glancing at the bird often but not looking at it steadily, this is so the bird knows that you are not looking at it like a predator does. Now slowly begin to approach the cage and as soon as the bird acts nervous or begins to move away from you in its cage stop and do not approach any closer during this session. Now all you want to do is to talk to the bird in a calm voice. It does not matter what you say but include thing like good bird or other words of praise for it, I find it to be helpful to think about how I want this relationship to be and talk about that and things I would like to do with the bird. When the time is up say good bye and leave. Do this the same way from the very beginning and in a short time you will be right at the cage.
Once you have reached the cage, you can simply begin the session with hello or what ever but you now will offer the bird a treat through the cage bars. It could be a unsalted, roasted peanut, a sliver of carrot or apple. Three pieces should get you through the 15 minute talk. If the bird takes the treat from you, great. If it doesn't take the treat leave the treat right there for it and continue, at the end of the session say good bye and leave. Once the bird is taking the treats from you then we begin the next step which begins with you starting your talks by opening the cage door. You offer the treat as before and if it takes the treat from you then the next treat is offered so that the bird must step up onto your hand to get it If it steps up and takes the treat say Step Up. At this point you have started to gain the birds trust and you can start to move about in the bird safe room with the bird on your hand. Make sure that you save a treat to give the bird when it is time to put the bird back in the cage.
Will go into giving the bird free time out of its cage in my next post here.
While I am happy to try to help you with your bird, you really owe it to the bird and its owner to try to find a way to reunite them if it is possible, and it is really where I should have started my reply. birds bond to their owners and when they become separated from them they grieve the same as you would if your child got lost. The owner of this bird could also be just as frantic over losing this bird as you would be if it were your child. I know that I would be if I lost one of mine, because I nearly did lose my African Grey Parrot, I was lucky and recovered her.