My 11 year old son has been caught stealing twice in 2 weeks at the local shop.
The things he has stolen he did not need one was a plastic gun that he had the money in his pocket for and the next occasion was a packet of sweets.
I have kept him in the house and asked him why he has took these things but he just says "i dont know" this is a very worrying time for me and I don't know what to do.
Should I be seeking medical advice? Or could it be a phase he is going through?

It could be any of a number of scenarios. Actually a plastic gun and a packet of sweets are things that most 11-year-olds would want so that kind of makes me question kleptomania. Now if he were taking things that would be considered totally worthless to an 11-year-old boy, such as perfume or lipstick, then that would make kleptomania more likely. He may just be testing his limits. Continue to monitor his behavior and impose consequences for unacceptable behaviors. If it continues in spite of these measures then medical intervention may be necessary.

Children shop lift for two reasons, someone else put them up to it and they think it'll be fun. OR they are seeking attention. Are you in a relationship? Have you had a big falling out with someone the boy cares about very much. Your son needs your attention and your reassurance that everything is going to be okay. Tell your son you love him and that if there is anything he feels he needs so much that he would think about steeling it, ask him to come to you and give you the chance to get it for him because you love him and do not want to see him getting into serious trouble.

Don't shout and yell about the theft. Talk calmly, let him see you are very upset and that these actions are very wrong and that you will do anything you can to help him because you love him.