I'll offer my thanks to Norman for apologizing (since it was me who spotted his error and informed him). I'll also try to clarify my answer which I gave to your other post:
With a 'normal' prison sentence, the offender is usually released well before the end of the actual term specified by the court. However, an IPP is effectively the same as a life sentence. (The only real difference is that someone sentenced to 'life' will always be on licence after their release from prison. Someone who is released from an IPP can apply to have their licence period ended).
As with a life sentence, there is no definite end to an IPP. However, the court sets a 'tariff' which defines the minimum period which the person must remain in prison. If, at the end of this period, the parole board believe that the offender no longer presents a risk to the public, he will be released. However, if the parole board believe that there is still a significant element of risk, parole will not be granted. (Theoretically, someone sentenced to an IPP could spend the rest of their life in prison but, as with life sentences, this would hardly ever happen).
As stated, the tariff defines the minimum term of imprisonment. Regrettably, from your point of view, your son won't be released until he has served at least 5 years in prison. (Like all prison sentences, this includes any time in prison, prior to sentencing, while on remand).
Chris