HELP! Asperger's teen with Internet Addiction!

I am looking for advice to help me with my 17 yr old son, who was diagnosed with Asperger's Syndrome and ADHD when he was 5. Alex spends most of his time on his tablet and I'm starting to think it may be becoming an addiction. It's even become a major distraction at school, where he barely passed a few of his classes this quarter. I have tried redirect him to other things but he can't seem to put it down for long before he's logged on again. When we are having internet problems and the Wi-Fi is slow or down, he gets very frustrated and impatient waiting for it to be repaired. Other parents out there raising children with Asperger's should be familiar with their tendency to be very focused at times, sometimes with all their attention on one thing. He has always been into different technology and is very skilled with computers. He hopes one day to become a graphic designer. I don't want to discourage him from that, but I am concerned about the amount of time he spends online. It's an all day thing more and more when I see him around the house as well as in the car. Does anyone out there in this group have any ideas to share with me that may help? I am new to Reddit and this is the first place I searched for. Any bits of knowledge, advice, or, just good ideas to gently guide him away from the internet for bits of time are welcome. He doesn't seem to be interested in anything I' ve tried. I am open to all opinion or ideas. Thanks for the help!!