sillysue it's easy to get caught out like this and you won't be the first parent to have made this mistake.
First you need to lock that phone down so that he can't access undesirable material. Phone up the network provider and get the parental controls turned on. If your provider won't/can't then switch to another one.
Secondly, secure your home network. With mobile devices so common, it isn't enough to secure computers - you need to secure the wireless network too. So a quick Google for the make of your internet router and 'security' and you will find instructions on how to access the security settings to bar certain websites, passwords etc. You can buy software to do this, but most routers have it built in. Go and ask on the Geek board here if you get stuck.
Now you need to deal with talking to your son about this, and I can't pretend to know anything about Aspergers so my approach may be no help. I would suggest first of all having a general chat about how there is stuff out on the internet which is harmful, and that sometimes people are curious and see stuff they shouldn't.
Then a couple of days later, raise the subject again and ask if he has ever encountered anything that makes him uncomfortable.Try to get into a bit of discussion about it. Might he tell you at this point, do you think?
I guess that then you need to confront the issue, having laid the groundwork, confront him about his phone. Stress that this is about keeping him safe. Are there any age appropriate books about sex that you could give him? I think puberty is a very difficult time for boys as so much info in popular culture is aimed at girls, and those burgeoning feelings and desire for information that boys must have leads them to seek out this undesirable stuff without really knowing what they are getting into.
I hope that helps and you can get some good advice from others on how best to approach your son. But the tech bit is easy - just get it sorted quickly now.
I must just say finally TeacherD that I'm glad your household is so organised on this issue, and that you are certain of the right way to approach the problem. This might not be the right place to air those views so strongly though. It's easy to be wise after the event.