have you talked to the parents? they will be able to tell you what is his "normal" behavior, and what is defiant, and what is both, and hopefully what works for him. unfortunately, there is no "one size fits all" for autism.
My son is 6 and nonverbal (uses ASL), and he has issues like "joe" at times. He's in a special ed class, and I work w/ special needs kids in a medical facility. the taking off of shoes and socks is pretty typical to kids w/ sensory issues which usually goes hand in hand with autism. I would let him take off his socks for a short period, and try to put on a different pair...ask parents to send extras. It may be the tightness of the socks that bother him, or the color, or the texture, or the weight. if you can have the parents send extras to try, you may be able to get his cooperation. sometime my son takes off his socks when they get sweaty, because he has an aversion to feeling wetness on his body...he is more than happy to put on a new pair of dry socks, but trying to get him to wear the ones he just took off is a nightmare.
Does he have physical or occupational therapy at school, is that something that is available in your school district? Things that work for calming can include a spinning board, weight vests/blankets, pressure vests/blankets, brushing with joint compression...but you would need special training to do these interventions..would your district provide it?
does he like/follow books? Social Stories can help w/ walking thru proper social behaviors. Most kids with autism don't understand the social cues we take for granted and it can cause a lot of frustration which leads to behavior issues and tantrums.
One thing that really helps my son at school is a schedule board. they start the day mapping out what activities they have to do 1st thing in the am, and then as each activity is completed, it is removed from the schedule. this gives order, and routine, which is CRITICAL for kiddos with autism. good luck!