Your husband has pleaded guilty so there is very little you can do now , sorry.
Changing solicitor will do nothing as it is now down to the Judge to decide on the sentence and the solicitor has nothing to do with it.
Section 18 is a very serious offence , only 1 step down from attempted murder !
A prison sentence is always given for a S18 unless there are VERY exceptional circumstances , as your husband pleaded guilty he will get 1/3rd off whatever sentence he recieves , so if he gets 6 years it will be reduced to 4.
Then any sentence of 4 years or less has automatic release on licence at the half way point.
So let us say for example that your husband gets a sentence of 4 years , the 1/3rd reduction for a guilty plea reduces that by just over 14 months so it is down to 2 years 10 months then the halfway release will be at 2 years 5 months.
I am not saying that 4 years is what he will get just trying to give you examples of what can happen. To get a better idea of the possible sentence we need more informaion about what happened and what led up to it.
But I will say that 6 years for a 1st offence does seem on the high side unless it was a very serious case. But if as you say he hit someone so hard that the other man is going to have a permanent sight defect which will effect his career that could be classes as very serious. Please come back with more information and I hope we can be more helpfull.