I don't think an aquatic wheelchair is necessary, but maybe tiny fish crutches would help him swim better.
Its probably genetic, but as long as he is swimming, its probably fine.
No guarantees about whether or not he will grow. But, as long as he is getting food and given the chance to grow (which it sounds like he is), then you can just wait and see what happens. I have gotten a couple of runt platies before but they still lived normal lives.