Sorry, but exdc has got it spot on. A suspected shoplifter does NOT have to leave the store in order to be charged with and convicted of theft. This is another urban myth.
I've seen all the comments about store staff being properly trained - however, irrespective of his proper training, he could well have mistaken the OP's brother for a prolific shopkeeper or reasonably believed that the OP's brother was in the process of committing a crime.
Unfortunately, the devil is ALWAYS in the detail in these cases. Whilst it might appear that a serious assault has been committed, the question of reasonable force will come into it. The supermarket worker may not have intended to bring the OP's brother to the ground, may have tripped etc. ON the other hand he could have been looking to be a hero.
From what we see on the posts here, we are never going to get the fullest picture.
The advice that has been given, eg, get the CCTV and see a solicitor is the best that can be given under the circumstances.