Sons fear of being alone

Hi,
My almost 11 year old son is afraid of being in a room by himself. If we are downstairs and I just take something upstairs he has to follow me. If he is playing with his younger brother upstairs and his brother pops downstairs for something, he goes beserk once he realises he is alone. If he is in an adjoining room, then all other doors need to be shut. He wont pop upstairs to get anything without being accompanied. This has been going on for a while - I thought he'd grow out of it but it seems to be getting worse. If I try to talk to him about it, he clams up and says theres no problem. His father and I did get divorced a few years ago but we are both with new partners and the whole situation is amicable, so I dont think this is related, do you ? Please help as I am very worried.
Thanks DoDah

[JSGeare]

Cut to the chase and get into counseling. You'll drive yourself crazy trying to figure out what it is and then what to do about it. Your child does seem to have serious anxiety and it does SEEM to be related to abandonment. But he's also getting attention through his behavior -so, call in some professional help and please let us know how things go.