Hi,
congratulations on your new friend!
I am very new to these birds too, I have a 5month old little boy and I have only had him for 2 months now so as you can understand I am still learning. However I did do alot of research before and after I got Asteri from the breeder, books and online as well as this great forum - all have been very helpful!
So although I am new I am happy to offer you some of the information I have read and learnt about.
Keep his water fresh, change it at least twice a day & definitely before you cover him at night to sleep.
Make sure you remove fresh food (fruit vegetables etc) after a few hours in his cage because they can spoil and make your bird sick and they also attract little fruit flies.
Constantly give your bird attention, always aknowledge him when ever you enter the room, take even just 20 minutes a day where you spend it one on one with your bird without distraction.
Allow him some time out of the cage a least a few times a week if you can not do it daily, although a few hours a day is much better than many hours on just a couple of days - routine truly is the key here.
Have his wings clipped as he is an older bird and is clearly not trained, this will prevent him hurting himself and also allow you to have a much less stressful interaction with him while he is out - this will make it easier for you to train him.
Be kind to him, dont yell at him or get frustrated with him. He doesnt know you and doesnt know why you are angry at him - when he bites you or runs away from you it is because he is protecting himself and is unsure or even afraid.
THE WAY TO HIS HEART IS THROUGH HIS STOMACH

!
remember this always.
Dont just put his food in his cage, offer it to him with your hand - he will quickly learn that coming to you is a good and yummy thing to do.
Food:
If he was eating a seed-only diet (which is not healthy and quite fatty) then it will quite hard to switch him to parrot pellets - there is some great advice and tips on how to do that if you search the forum. Pellets are great because they are sort of a piece of mind insurace - they supply all the daily requires vitamins & minerals. Seeds, especially sunflower seeds, should be limited to once or twice a week as treat and for variety. So try and cut them out of the diet eventually. These birds prefer fresh fruit and vegetables anyway so if you offer him that then the wont miss the seeds at all - he will only notice when you replace the seeds for pellets.
DAILY - offer a variety of fresh RAW fruit and vegetables (wash them well before giving them to your bird).
NEVER GIVE HIM - Avocado, Chocolate, Onion, Garlic! These foods are poisonous, also the seeds of apples are also toxic.
They love apple, mango, pumpkin, corn on the cob, carrot, summer fruits, snow peas, peas, broccoli, zuchini, unsalted peanuts, almonds, watermelon, etc etc. Greece has great fruit and vegetables..he is very lucky, let him taste it all!
Also you should probably offer him a boiled egg once a week for protien. put some of the boiled shell crushed up with it in his bowl.
ok, thats pretty much everything I know!
I hope this helps you!
= L =