Being rule-governed has plusses and negatives. When it's hard to make sense of your world, having clearly defined rules is comforting because then you know what to do. But when others don't follow those rules...
A way around this is by doing social stories with your son about kids not following rules. He probably has not idea why another child would not follow the rule. The "Theory of Mind" or as Temple Grandin calls it, "universal mind" aspects of autism spectrum disorders means that whatever the person has in his mind, he assumes everyone else also has that same thought/feeling/intent. Your son may honestly not understand that not every child knows all the rules or
that sometimes kids forget. Social stories are good for getting those sorts of points across.
A social story would also help your son know what to do when another child breaks a rule - what he should say, how he should act, who will take care of the infraction, etc.
A couple other aspects of autism/Asperger's make organized sports difficult but still worth experiencing.
Many people with ASD have poorer fine and gross motor coordination; they may appear awkward, have an awkward gait when running, have trouble with fine motor skills such as handwriting. Then there's the motor planning issues inherent in sports.
When you think about it, there's a lot involved in accurately kicking a soccer ball. Just try
to explain step by step every small action you'd take when you try to capture a ball, line it up and kick it into the goal.
Autism is a processing disorder. Most ASD people have trouble processing more than one element at at time. Your child may be able to look at you or listen to you but not effectively do both at the same time. Team sports involve a lot of simultaneous processing.
Not only do you need to be aware of where your own body is, control your arms, legs, feet and head but you have to be aware of where other people are at the same time. This is further confused because some people on the field work with you and some against you. Then you need to keep a sense of direction in your mind so that you don't score on your own team.
Compounding this might be the sun flashing in your eyes, temperature and wind, the icky feel of mud on your hands and knees when you fall on a rainy day. And, it's not silent out on the field. The kids yell. The coaches yell. The parents yell.
Auditory processing is a weak area for most people with
Asperger's/autism and the ability to process when you hear decreases in noisy situations or when there are background distractions.
If your child has a lot of sensory issues, he might feel calmer if he wears a weighted vest, small wrist weights, a snug neoprene vest, neoprene biker shorts under his soccer uniform or similar apparel to help calm his overstimulated senses.
Prep work such as practicing the rules of the game alone with your son, taking him to the soccer field when no one else is around so that he can accustom himself to being there, practicing the coach's soccer drills before practice time can all help ease the way and make your son feel more relaxed during team practices and games.