The sentence is perfect. It doesn't imply that he has now stopped hoping. It also doesn't imply that the event has not happened: "He has been hoping for six months that it will happen. Little does he know that it happened two weeks ago."

I believe it is better to use the subjunctive here rather than the future with "will". Subjunctive is used when talking about events that will probably happen, for example when you want it to happen or when you imagine it can happen. The form of the subjunctive is very simple, it is the base of the verb, ex: for the verb "to happen" the base is simply "happen". In you case you can then simply write: "He has been hoping for five months that the event happen"