I highly recommend the book Taking Care of Myself by Mary Wrobel - it is a social story book on puberty and hygiene and it covers masturbation. The counselor will likely only be effective if they are a behaviorist. This can be treated with behavioral strategies. Does your son have a behavioral therapist?
It seems to me that your son's counselor should be able to help you with ideas. It may be something that will lose it's appeal after time but I just don't know. In the meantime, I would keep the computer somewhere that you can keep an eye on what he is doing. My autistic child is a girl so aside from hour long showers, I haven't seen or heard anything that makes me suspect masturbation. Good Luck.
This is a pretty interesting case because of the autistic disorder. Making this something that really needs to be treated by a professional. I would say to any parent it is normal for a teen his age to masturbate (wild hormones and all). But doing it in the right places is some what harder to teach your son and does like I said require a professional.
I am a computer professional myself and have worked with parents insecuring there computer and pornography blocking. While anything is defeatable with the right mind you can make it as hard as possible. My parents tried to lock me out of the computer several different ways and each way I would beat it. Not to really go look at porn but because I wanted to defeat the lock.
Can you tell me what kind of computer you have and what the operating system is? I would be happy to offer some ways to secure your computer.