One of my difficult children is just now to the point of waking up at night (sometimes), at 9 yrs old, yes pull-ups get expensive & she's into the big kid ones now which cost twice as much but all of her doctors have told us not to worry & definitely not to punish or make a big deal out of it. She's now used to waking up in the morning and running thru a quick shower if she's wet. If her bed is wet (she has a plastic mattress cover which I highly suggest) she'll strip the sheets & blankets herself & take them to the laundry room. She is not Autism Spectrum Disorders (ASD) but has very similar symptoms as far as development goes. So don't sweat it, it may be a bit more work but you don't want to cause him to stress out about it & like most others have said, event NT kids often still wet the bed at 4 (my NT 4yr old turns 5 in July & just now is fully potty trained, he was just stubborn).