Update: Hattie came to us almost a month ago unable to stand. His feet were always closed and whilst he could flex his feet and move his legs he wasn't able to stand, perch, walk or use them in any way.
After a lot of research and expert advice, we started him on multivitamin and calcium supplements and he has been steadily improving. He began to relax enough that the feet would open in his sleep, then they would be open and grip while awake. Now although he still has trouble walking more than few steps without stumbling, he can perch!! He can also sleep like a normal bird instead of sleeping like a baby, and is so much more active during the day.
He also had no idea what food was and it's been a long process to coax him to eat solid food. He now he nibbles all day long and we only need to hand feed occasionally rather than 4 times a day!
Although his feet still clench and he falls now and then, the improvement in the last month has been amazing to watch. He has just started trying to fly which is a great indication of his improving health. Hopefully soon he will be fit enough to go into a larger enclosure with other red loreds so he can start making birdy friends.