We went last year and the rules had changed and all we needed was to show the blue badge at city hall to get the pass , it lasted for the entire duration of your stay so you don't need to go everyday and TBH it is brilliant . The year before we had to take medical evidence ( letters ) so this was much easier. You don't go in the regular queue and rarely have to wait more than a few minutes . For the busier rides they give you a time to go back .
If you have a blue badge take it with you ,that's all you need to get a mobility pass which means you get on all the rides quickly . Your son will be eligible as well but the passes for autistic people aren't as good IME as the mobility ones . The Disney website has a very good disabled guide on it with all the info about what you need to take to get a pass. Have a lovely trip.