I'm reading Dividing the Spoils currently and throughout the search for a single successor Arridhaeus is constantly referred to as simple. Without giving too much away (I haven't gotten far in the book and honestly have no clue what happens to anyone although if I had to hazard a guess I'd say he's probably going to end up being killed) do historians today have any idea if he really was just dumb, or mentally retarded, or maybe even autistic?
It's very hard to say. Our ancient sources are pretty thin, and most of our ancient biographies of Alexander come from well after his life. Elizabeth Carney has an extensive article on the question called "The Trouble With Philip Arrhidaeus" in Ancient History Bulletin 15.1-2 that goes through the ancient sources. She concludes that he was mentally retarded in some way, though it's impossible to know if it was genetic or acquired, or to say precisely what the extent of his handicap was.