if your son has a learning disability perhaps you could look into a special needs stroller, i know a friend of my mums has a special needs child who is in a stroller still at 9yrs old, its a lot bigger than a normal stroller so im not sure how much it would cost tho, but perhaps you could get some financial assistance from centerlink or something to help with the cost?
i also know of a child i used to care for that had global delays (so he was basically a 12month old mentally but he was 5years old physically) his mum used to push him around in one of those $30 big w strollers. i dont think they have a higher weight limit on them but i guess if they dont last long, at least they arent very expensive to replace.