Can't fly?

I recently adopted a 12yr old IRN when his owners couldn't keep him due to noise and small children. Indy is a lovely bird, talkative and happy to step up and hang out on my hand. A bit cage aggressive, but he's all bluff.
The big problem, Indy won't fly. For the last three years he was kept in a smallish outdoor aviary, enough room to flap and glide but not enough to fly. Prior to that he was a house bird with clipped wings. He's very confident walking and climbing.
I have had him inside for the past two months so he and I could bond, and is allowed out of his cage for 2-3 hours a day. I've just started putting him in the outdoor aviary during the day for some sunlight. The two times he has attempted flight he has just hit the ground. No lift at all. I've tried encouraging him to keep trying by holding him a foot or so away from his cage and tempting with treats. He looks like he reaaaallly wants to, but is just not confident to do so.
Needing some tips to help him build his muscles and confidence?
My conure was also hestitant to fly after being in a small cage all her life, but she's a brilliant flyer now and follows me around the house begging for treats.
Thanks in advance!

In the Amazon Forum is a huge Thread Titled: I Love Amazons - ... Within that Thread on page 1 is a list of all the segments. Locate the one that speaks to refledging an Amazon. That segment will guide you step by step on introducing flight.

Understand that your Parrot likely never learn to fly when very young. The process begins with developing the flight muscles, heart and air sacks. Then moves on to working with landing and as a result confidents.

Your first step is to have your Certified Avian Vet (CAV) complete a detailed examination to determine if your Parrot is currently healthy enough to learn how to fly and has the heart health to support it. Understand that near all Vets only check for heart rate and not the physical condition of the heart.

Commonly, your CAV will recommend to start the process, but set-up a series of examinations during the process.