I don't have a boxer, but I have a cane corso called Kaddie (6months old) that had similar problems. She's never been scared of people....but noises or strange objects would send her in to a frenzy! She wouldn't walk past dustbins, bicycles (even stationary ones), parked cars etc, and she was terrified of the hoover and when I tried shaking a can of coins to stop Wicket barking, Kaddie just freaked out! The kids could shout and run around to their hearts content and it didn't bother her, but if they got their toy cooker out, or a dolls house Kaddie would run and hide.
I wanted to enrol her in the Kennel Club Good Citizen scheme, so I did the puppy assessment course first and it really helped her....they got her used to everyday objects and it was really successful. She recently got her bronze award and is working on her silver
Sorry for the long rambling story
Is there a similar training course in your area that you could enrol your boxer in? It might help him to get some of his confidence back. I wish you lots of luck xxx