I worked as a store detective many years ago and I could not tell you how many kids I caught stealing. For boys, the age range for the second "sticky-fingered stage" is between seven and ten. For girls, it's about eleven to twelve. (The first stage is when they are very young - before they really understand the concept of personal property.) The best thing that can happen to one of these kids is that they get caught immediately . . . otherwise the behavior usually continues. Unfortunately, most kids get away with it because store detectives prefer to focus on adults (they steal more expensive property).
In my interview with the shoplifting kids I would catch, I would always ask them what they planned to tell their parents about the new stuff. (My mother would have been VERY suspicious if my sister and I had come home with things that she hadn't bought for us. I wanted to know what these kids were telling THEIR parents about this stuff, so I could let the parents know what the deal was.) The kids told me that either their parents wouldn't even notice the new items OR that they intended to tell their parents that their friend (and fellow co-shoplifter) gave them the stuff. (Most kids don't steal solo. Shoplifting is a buddy activity - which is why I'm worried about your son.)
My STRONG advice to you is to confront this head on. This little boy needs a dose of embarrassment NOW . . . or he's going to be dealing with the criminal justice system within about two years. If his parents are reasonable, go to his house with your son and politely and calmly ask for them back. Do NOT ask him if he has them. (You never want to use a yes or no type of question with a kid - you're just inviting them to lie to you. Just tell him that you need him to go and get the Pokeman card right now, please, because your son only wanted the friend to play with them - NOT keep them forever.) 90% of the time, the kid will run and get them. Then perhaps you should lay down the expectation that he is not to take anything from your home or your son that he did not bring UNLESS an adult gives him permission.
If the other parents are NOT reasonable, wait until the boy comes over to play the next time and then sit down with both boys. Very calmly explain to the boy that you and your son are very disappointed by the fact that he took some of your son's Pokemon card. (Don't ask him ANY questions. Resist your impulse to ask him IF he did it - you already know he did. Don't help this kid turn into a liar as well as a thief. Most especially, resist your impulse to ask him WHY he did it. Kids have little to no impulse control at this age. He took it because he wanted it in the moment and wasn't thinking about any future consequences.) Let him know that you care about him, and he is your son's best friend. Explain to him that he needs to return the cards immediately before he will be welcome in your home again. THEN, very calmly and very sadly explain that you expect that he will never again take anything from your home or anywhere else that does not belong to him; otherwise he and your son will not be allowed to play together anymore.
I know this sounds harsh. Especially for an eight year old, but his parents are dropping the ball on this sticky fingered stage. If YOU don't step in, this boy will continue to do this . . . and he will start doing this in stores, and he'll probably do this in front of your son (and perhaps even WITH your son at some point).