Unfortunately neutering a fearful male dog can make their behaviour worse. He was showing aggression on lead because he was frightened and felt trapped, off lead he was probably still worried but knew he could flight if the going got tough, so didn't need to fight (show aggression), also the fact that he humped a lots can be a fearful behaviour. Castrating a dog stops the production of testosterone which put simply is the hormone of competitiveness and bravery, so he is now more fearful than he was before, he has also found a great way of relieving himself of his fears, he uses aggression, and either the threat moves away or you move him away, either way he gets the desired results.
As for curing him it will be a long slow process, of stopping the behaviour and confidence building, to be honest you really need some professional help, ask round for recommendations in your area.