To be honest, very few ponies make equally good lead rein and first ridden ponies, let alone ones which are rescue cases/are inexperienced. A good lead rein pony is trained to take its cue very much from the handler and to be very unflappable, to the point of almost ignoring what is going on in the saddle (so they don't overreact if banged with the legs, clumsily jabbed in the mouth, bumped up and down on in the trot etc, all things that can happen when little ones are learning). Also their first instinct if something is wrong, e.g. rider is unbalanced or handler hesitates, should always be to stop and certainly never to move off without someone beside them! Whereas while a good first ridden does need to be tolerant of inexact aids and not over-reactive, they should be more forward going and attentive to the rider, sensitive to the aids rather than to what's happening on the ground.
So it's hardly surprising that your pony is a bit confused and looking to see where you are, that you now want him to not follow you and take his aids from his rider, when you've been teaching him the opposite the last few years. This isn't your fault, it's nothing you've done, some very clever/saintly/well schooled ponies can do both jobs and switch between them with ease, but TBH a lot can't, and it does usually require a more experienced child or small adult rider to get them going off the lead rein at first, and some confident teaching/supervision from the ground as well, to reassure pony that yes, legs on means go even if handler isn't beside you etc. Almost like breaking them in again from scratch. And you may find that if you do getting him going well off lead rein he doesn't necessarily switch back to being the super unflappable lead rein you had before if your son wants to ride him again.
I'd get an assessment from an experienced and trustworthy instructor (ideally one that has access to a competent child rider or is small enough to ride pony themselves) to see if s/he thinks with some lessons and input he can be turned into a good first ridden, but if he was mine I think I might be tempted to leave him doing the job he is good at and see if I can find a nice ready made first ridden for DD to get her confidence on. When they first start to ride off lead rein is always a tricky time and the old saying is that green on green makes black and blue, while I certainly hope this isn't literally true in her case she probably would enjoy herself and learn more on a more experienced pony. If this means loaning out pony or finding a sharer so be it, you shouldn't have any difficulty finding a great home for a good lead rein, or maybe DD could have lessons on riding school ponies, and just ride your pony on the lunge or lead rein?