I dont think hes addicted. I think hes an 18 year old boy. Im somewhat younger then majority of you guys, (sorry its true!) I dont think computer gaming was anywhere near as accessible or exciting when you were 18 compared to now. Hell at 18 I was moving off dial-up and moving onto "high speed" internet and trust me once I had that connection I spent pretty much 70% of my day sat in front of a computer, 20% drinking (drinking age in the UK is 18) and the last 10% sleeping, or if the bars were closed make that 30% sleeping.
Everything has access to internet now, consoles, computers, phones, tablets, and theres games readily available on all those devices too.
I dont think theres any hidden meaning behind his constant gaming/sleeping, I think its pretty typical of someone in his age group, hell my 16 year old brother in law pretty much lives that same kind of life but is slightly more involved at school though I expect that will change in a year or 2.
Theres obviously step-parent/step-child issues but again I dont think thats related to the gaming I think thats something entirely separate but does need to be worked on. Unfortunatly its a bit of a catch 22 situation, no trust from one party leads the other one to act out more because...well why bother if they wont believe you anyway? Been there done that myself, I ran rings around my parents and had them pulling their hair out.
If a 28 year old was spending a lot of time online playing video games, most people wouldnt think too much of it, thats an adult after all. at 18 your son is not a kid, and suggestions like taking his computer away, putting it in the family room, limiting his time, blocking the router...those are things that would be great suggestions for a kid...at 18 your son is an adult. Doing any of those things wont make a slight difference I promise you. Instead of a tug-of-war, catch 22, going round in circles of retaliation spiral....just sit and talk to him about your concerns, dont talk down to him, dont baby him, just be straight and honest with him without it being you talking to him like a child and telling him off. Tell him how it makes YOU feel, how its affecting the family dynamic, how your concerned about him, his grades, his sleep, his mental wellbeing. You may just be surprised at the response you get.
good luck! (and belated happy birthday!)