Sacked after work injury

Hello all
I am wondering if someone can help with this query.
My son was working at a new restaurant in Central. On Sat he injured his back at work and went to doc on the Mon and got a sick note stating his injury. Upon arriving back to work on Thurs, he was fired and was told it was due to poor performance. He had never been notified in the past of this so was quite shocked.
Whilst discussing this, he was given a form to sign. He was not given any copies.
Does he have any basis to file a complaint? He was still under probation but was given the impression they were happy with him.
Before any judging, I know he should be posting this and not me. However, I think I am angrier about it. Please only helpful comments and no mama~bashing.
Cheers