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!