As I read it, your son was convicted of a serious assault (GBH? GBH with intent?) 10 years ago. You've now stated that he's been accused of little more than 'mugging' 10 years later. There's absolutely nothing there which indicates a 'pattern of behaviour', likely to lead to an IPP.
Before anyone can really try to make sense of your question, and then provide a useful answer, we need all of the facts. Did your son have any convictions prior to the incident 10 years ago? (If so, what were they for). Exactly what offence did your son plead guilty to 10 years ago? Has he 'been in trouble' in the intervening period? (If so, what for?). Exactly what has he now been charged with?
I'm sorry that this post doesn't directly help with your question but I can see no reason why a court would consider imposing an IPP on an offender unless there's evidence of a repeated pattern of offending. 'Mugging' someone for their motorbike, ten years after 'getting into a fight' doesn't display a pattern of offending which requires an offender to be locked up indefinitely 'for public protection'.
Please fill in the gaps and then, hopefully, someone here might be able to advise you.
Chris