T.N.
Neither one of my two sons (now 17 and 19, and both recipients of academic college scholarships) were fully potty trained until well after their 4th birthday.
My daughter, however, took her diaper off, climbed up on the pot, and was done with diapers forever at 18 months.
Point being, on the spectrum or not, every child is different. After you try all the tricks everyone swears will work, and he still isn't getting it, put it down for a couple weeks and try again.
And relax, potty training is WAY less stressful then teaching them to drive!
:)