My DC has no ASD diagnosis, but was late to use pronouns (at around 3.5 years) and would initially reverse them all the time (until around 4).
We conquered it by excessively and purposefully using pronouns in front of our child. So a conversation at the dinner table may have been:
Me: I, mummy, would like to have the butter. You, daddy, could you please pass it, the butter, to me?
DH: Yes, mummy, I daddy, will pass itt, the butter, to you. You, DC, would you like to have some some of it, the butter, too?
We did this whenever we were alone with DC and found it worked especially well at the dinner table and when playing board games. We basically had to 'show' DC how to use pronouns whereas a lot of other children just 'get' it.
Don't know whether this will work with a child with ASD, but may be worth a try?