Has hs mother ever followed up any of his 'excuses'? Been to a GP or pyschologist? I find it strange she just laughed it off with a 'your problem now' attitude.
Because if this has been a long term problem I'd be taking him to the GP for blood tests and if that's all clear then I'd see a child psychologist for his anxiety issues. And I'd talk to his teacher to see if they thought there were any problems or issues.
Good luck gettng to the bottom of it.