I'd get the mite issue dealt with as well as a fecal done if he hasn't had one recently just to rule out any internal parasitic load then just feed him once weekly. Just give him the best, chubbiest, healthiest prey around and as long as he's healthy I'm sure he'll come along for you in time. I know from a couple of our rescues that once they are well and eating weekly, they do seem to make great strides but it's always about just being consistent with their feeding and letting them gain back their weight in a normal fashion. I'd avoid twice weekly feedings as they never seem to be recommended and I'd personally worry about pushing him too fast and causing issues with regurging if he can't digest quickly enough between feeds.