He would have been treated better than how he was OTL, since I doubt Alexander would murder him in cold blood like the Satrap of Bactria Bessus did. It helps that Alexander got along really well with Darius family that he had captured some time before during the Battle of Issus. Alexander gave him a magnificent funeral after he found his body too, which shows that Alexander held no hard feelings against him.
While exactly what would happen is hard to say, Darius would likely live for awhile, however if the Macedonians throw a drinking party he might die. Those legendary drinking parties the Macedonians threw tended to be rather catastrophic to whoever or whatever pisses them off while highly drunk. As seen by the burning of Persopolis and years prior the split between Phillip II and Alexander that likely laid the ground work for Phillips eventual assassination. Also during the last years of Alexanders campaign he became increasingly paranoid and willing to murder his subordinates at the slightest whiff of trouble, which might put Darius at risk if any hints of rebels supporting his restoration crop up.
If Darius is lucky, he gets to enjoy freedom and retirement from the role of monarch he clearly wasn't fit for, a bit like Nicholas II was during the time after his abdication. Nicholas was reputed to have been very much happier freed of the weight of being Tsar, and Darius character seems to me like someone who would react in a similar manner.