You are showing your own integrity in mentioning your child's problem in a question to us, and I am confident your attitude and behavior will help remedy the situation. Sometimes behavior like shoplifting is an early indicator of more serious anti-social behavior. But, if so, it is usually associated with other indicators (like bullying, chronic lying, hurting animals, etc.). From what you write, it sounds as though your son's anti-social behavior is pretty much limited to this one area (although you do say that he refuses to take responsibility for his actions and tends to blame others). As long as it is restricted, I would deal with it by itself and try not to think of it as signifying a broader and more serious pattern.
But now what do you do? I always suggest starting with a fairly simple and hopefully preventive approach: don't take him with you into any store. If he complains, remind him that you are doing this for his own good until he learns the difference between his things and things that don't belong to him. When you come right down to it, a toy store can be an overwhelmingly tempting experience for a young child. They have so many things, and you want only one! What can be the harm . . .? So don't tempt him. If you have to take him with you into a store, make certain that he doesn't have with him any sort of backpack or satchel into which an item could be stuffed.
Second, think of some reasonable way for him to experience the consequences of his behavior. Even if he took the item back, check the price and have him give up something of equal value. If he has any concept of monetary value, either withhold or take from a piggy bank enough money to cover the cost of the item.
If you see this sort of behavior spreading into other areas of his life, talk to his pediatrician about it and possibly ask for a referral to a child guidance center.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.