Take your child to a counselor - a pediatric counselor. They will help you figure out if your child has a developement problem, if he's high functioning on the Aspergers spectrum, or if he's just going through a tough developmental stage.
this could be soemthing, or it could be nothing. It could be that someone in class makes him feel uncomfortable, or it could be that he is dealing with something personal. Has he lost a good friend, has there been a chagne in yoru family like a grandparent's death, a divorce, a job loss? Kids personalize things - if their friend's parents are going through a bitter divorce some kids may get fearful about their own family. If a bully at school was mean to him on the playground and then later got special treatment from the teacher on a completely unrelated thing if can make them fearful that the bully has power to harm them.
At age 11 his hormones may surge - and it's not just the hormones controlling puberty - but it's all hormones that surge att his age - so some kids become anxious, others get really shy. in 6th grade my daughter ate lunch in the bathroom stall - and I only learned this when she was already out of high school. She became super sensitive.
A counselor can help figure out what's going on.