>>> Yes my son committed a GBH charge but not to the extent of a section 18
The level of injuries required for a Section 18 charge is exactly the same as for a Section 20 one. The only difference is that Section 18 requires 'intent'. Quote (from the Crown Prosecution Service website):
"The prosecution must prove under section 18 that the defendant intended to wound and/or cause grievous bodily harm, and nothing less than an intention to produce that result, which in fact materialised, will suffice. A person ‘intends’ to cause a result if he/she consciously acts in order to bring it about. Factors that may indicate specific intent include a repeated or planned attack, deliberate selection of a weapon or adaptation of an article to cause injury, such as breaking a glass before an attack, making prior threats or using an offensive weapon against, or kicking, the victim’s head".
As has been stated above, if your son has pleaded guilt to a Section 18 charge, the judge can only sentence him on that basis. However if the level of injuries sustained by the victim were at the lower end of the range that constitutes 'GBH' (and there were no other factors, such as psychological ones, pushing the classification higher), 'harm' will be assessed as 'lower'. Your son's autism and mental health issues might also see 'culpability' classed as 'lower'. If both those conditions are met, the offence will be seen as 'Category 3', meaning that the sentencing range (without the one third discount for an early guilty plea) will be one of 3 to 5 years imprisonment. Knocking the third off takes that down to between 2 years and 3 years 4 months. The actual time spent 'banged up' is only half of the nominal sentence, so that means (assuming that the judge goes along with a 'Category 3' assessment), that your son will actually be 'out of circulation' for between 12 and 20 months.
If (and it's a very big 'if'), the judge sees the offence right at the bottom of Category 3, he can pass a sentence of 2 years (which, other than in very exceptional circumstances, is the absolute minimum sentence for a Section 18 offence) and then suspend it. However if the sentence is even just one day more than 2 years, it can't be suspended.
I've been following court cases for around half a century and I've hardly ever seen a suspended sentence handed down for a Section 18 offence but it does happen occasionally. There was just such a case before Ipswich Crown Court a few months ago, where the defendant's mental health issues were taken into account and his sentence was suspended. So there is just a very small chance that your son might receive a suspended sentence but it would be probably unwise to get your hopes up too much about such a possibility.