The parole board have made a recommendation it can be rejected by Amber Rudd
I'm afraid that is wrong. The Ministry of Justice (NOT the Home Office, so Michael Gove, not Amber Rudd) must comply. No politician has any power to change or reject the parole board's decision in this case.
If the decision was to move him to an open prison it would be different. That would indeed be a recommendation and would not be binding. But a parole board decision to release is final and cannot be overruled.
I don't understand why he has only been charged and prosecuted for a small number of the offences he allegedly committed
The additional offences did not emerge until after his conviction. There are a number of possible reasons why he was not prosecuted for them.
It may be that the police or the CPS decided that there wasn't enough evidence of the additional offences to give a reasonable prospect of conviction. The police telling the press they believe he committed over 100 rapes is a long way from them having enough evidence to prove beyond reasonable doubt that he did so. And prosecuting him with inadequate evidence makes successful prosecution later more difficult.
It is possible that they could only prove relatively minor offences (drugging, for example), in which case conviction may not have led to any change in his sentence. If that was the case they may have concluded that it wasn't worth taking it to trial.
It may be that they believed publicity around the case meant a judge would decide that a fair trial was not possible. I think it is unlikely this was the reason.
And, of course, it could be incompetence. And there may be some other explanation I haven't thought of.
I have no inside information but I would say the most likely reason is lack of evidence - either they didn't have enough to prove anything or they could only prove some of the more minor offences.
could Worboys technically be tried for the other offences
It depends on exactly what happened but, based on the information I can see, I believe he could still be tried for the other offences provided the judge was happy that a fair trial was possible. There is no time limit for prosecutions for serious sexual crimes.