Help! my dogs turned agressive after castration?

hello, i have a brown and white border collie he is very Intelligent. he was always a wimpy friendly dog. 4 weeks ago we had him castrated and since then he keeps going for other dogs. he had never done this before. whats going on. a friend suggested his hormones may just be settling down is this right?. he also barks more than he did before. will this all calm down, what can i do to ensure it doesn't continue, any help greatly appreciated, thanks