Perhaps a differnt form of intervation is needed. You know he is going to take that little bit of time to play with it in an "I told you I will still get to play with it" manner. This is when you take the toy from him and put it away yourself. It will be hard and you will feel like a meanie but kids need guidance and you will be guiding him if you do it this way. Also, offer more points for asking for items that belong to another person. Make the reward more desireable. If he still doesn't do it then he doesn't get the points, if he still continues do take things that don't belong then look at his prize list. Don't use demerits for this.. only use demerits for something such as hitting, biting, lying, name calling and such...
I take my kids every month to Toys r Us and they write down (or for my little ones I write down) the things they want that most. Then we plug it into the prize list. This really, REALLY works, because they want that toy, or gadget or item. I even offer things like $5.00 for 100 points, double point saturdays on every 3rd saturday, help a sibling-earn their points too, and last but not least combination prizes for combining points for things like Chuckie cheese, bowling, rollerskating, carnival, and local waterparks.
In the beginning I played a softie when I would give my kids the first try to make the best choice with my kids and one day I had enough of feeling like I was giving out too many chances so I set up a posterbaord with the rules and demerit list. My son said to me one day after instantly giving him a demerit, " hey, you didnt give us an extra chance" and I replied with, " I don't have to, it is set up so you can view it right above your check list". After that they knew not to give me the "extra chance" spiel.
After doing these things it should start working well. Keep in mind that the rules you have set up other parents may not have set up, and other kids may influence his or her behavior. As a last resort, take him out of a situation where you feel he may not do his best. This is a learned behavior and it will take close to 2 months of constant and continual reinforcement from you and those around you. If it takes less then consider yourself lucky! If you need any additional help you can write to me. 
Regards,
Kulita
Mother of 4, (Lilly-11 bipolar, Olivia-9 Aspergers, Ethan-6 ADHD & Aspergers, Bradley -4 ADHD), From 2003-2008, I ran a special needs daycare, May 2008 I went graduate school to complete my degree... still in grad school 
Mother of 4, (Lilly-12, Olivia-10, Ethan-7, Bradley-5). Just graduated Grad school. YAY ME!!!