My son is being hared for filing Workman's comp.?

He works at a grocery store halling carts. Rule is no more than 5-6 carts at a time. He's pressured to have the entire lot cleared in a short time so at times he halls more. He sprained his back which they caught on tap halling 19 carts. Someone else had helped him line them up. He asked for lite duty which required a doctors note of which my drs office doesn't handle workmans comp. His mgr told him they prefer to have their employees go to a particular hospital er which is where we took him. They later told him that he lied and said he only halled 6 carts when clearly the tape showed more. He claims he didn't injure himself at the time he halled 19 but injured himself when he halled 6. The one mgr even said back injuries don't normally show up till the next day so how can they see if the injury was on tape. He's not trying to make $$ off of them, just wants some lite duty till his back gets better. Mgr wrote him up & harres him ever since. What to do? Can his union help?