My 3.5 year old son is obsessed with Spiderman comics, games and movies and he thinks he is spiderman, he even has a 'girlfriend' in school who he calls Mary Jane . Is this normal? Or should I be concerned?
This role playing is normal, common, and essential for skills building. In imaginative role play children get to practice dialog, politeness, meanness, being the authority (parent), and all sort of other social interactions they have been exposed to. They also will seek out and gain knowledge on the subject of their role play to enable it to be more fun and accurate.
Spiderman is pretty famous around our house as well. My son and nephews frequently play as Spiderman.
If you feel he is too obsessive, if his focus on Spiderman causes disruptions or inappropriate behavior, start by coaching him on when he is free to play Spiderman and when he shouldn't. If it persists as a problem after this, find local help to advise you; a teacher, or doctor, or counselor, etc. But remember not to stigmatize the play. Keep a positive tone about the play and have negative comments about inappropriate times. An example from another context: "I love your singing son, you are so good, but let's not sing at dinnertime."
My son is the same, as was I. It's pretty normal. So normal that when I showed up to daycare with my mouth duct taped shut (because Spider-man doesn't have a mouth), the social workers understood. The teachers still had to call them in- probably some kind of policy regarding duct tape on children's mouths- but it was more hilarious than anything else, apparently.