Whatever it is, make sure it's sturdy. Toys that come into our house are subject to a lot of abuse. If it can be ripped apart, it will. It will get chewed on, stomped on, tossed around, thrown downstairs, left out in the snow, and possibly water-boarded. Small parts will wind up in his mouth.
Sturdy doesn't have to mean expensive. Take, for example, the Dump Truck Issue. We started out with a plastic Tonka. It survived years of play by my older son, but the plastic was maybe a little too thin. I suspect that exposure to the elements may have weakened it, but it wound up breaking into pointy shards after a few months with Ben. I decided to replace it with STEEL and bought this one: (don't buy it) I thought for sure this one would last, but it only managed about a week. Now the sides are all bent in, and it's like the metal edges have been worked over with a whetstone.
Ask the parents. Every kid has obsessions, and ASD kids can take it to a whole new level. Thomas the tank engine and Winnie the pooh have been pretty popular and there are lots of branded things to choose from. Right now, we're diggin' on Curious George.
If you buy clothes, make sure they're comfortable and easy to put on and take off. No tags is ideal. Better ask the parents what fabrics the child will tolerate and how they are with zippers and buttons.
Don't go by the age level on the box. A puzzle geared towards your standard 4 YO would vex my kid, and I don't want him vexed.
Take the kid's sensory needs into account. Some kids are sensation seekers, and some avoid them. Some can't stand loud noises. For us, things that light up, make noise, and move in response to an action (like button-pushing) are good -- particularly if the response is predictable, rather than random. Rocking horses, mini-tramps (preferably with a handrail), swings, and beanbags can be good.
Those are some guidelines. Here is a list of toys suggested by a speech therapist. I looked it over, and I think it's better than what I would have come up with.
List of Toys for Autistic Kids
Good luck and happy holidays to you.
Oh, Along Came the Bird has an awesome gift guide...check it out.