I've never dealt with this problem and can only imagine your frustration! I've heard that pj's with zippers in the BACK can be helpful with this issue. I did a websearch and found this website that sells them: http://www.ezzznights.com/Pajamas.html
I'm sure you can find back-zipping pj's elsewhere too, but just wanted to give this suggestion. Another thought - have you tried positive reinforcement? When our nearly 3yr old was resistant to potty training, we bought a bunch of gifts and wrapped them in pretty paper. We put them in a big basket in plain view and told her that every time she had a success on the potty (in your case every time your son does NOT pull his diaper off), she could open a present. This really worked for us because she loved opening gifts and wanted them so intensely that she became motivated to do what we wanted, but it felt like "her choice" (which is very important to 2.5 year olds!). Whatever your son wants or desires, use that as the reward for his good behaviour. When/if he isn't pulling the diapers off anymore, you can taper off with the gifts/rewards...1 reward for a whole week of not doing it, then so on. I'd also enlist his help in cleaning that bed, every time, gently saying something like, "oh look what happened because you pulled off your diaper. the bed is messy, now you need to help mommy clean this up. i'm sure you'll try harder next time to leave your diaper on and keep the bed clean!" he's not too young to know that his actions have consequences.
Best wishes!