Although I have girls now, in the past I have had male dogs and they were left entire. I never had a problem with them humping or being aggressive.
I had only one occasion when my GSP got the scent of a bitch and was offfffffffff, at the time I was quite scared but I then learnt to notice the signs and take the necessary precautions

(take him on another route).
I think a lot of vet practices (particularly the large ones) encourage owners to get their males castrated early as it makes financial sense for them.
My vet (a one man practice) said to leave it and see how the dog copes with his sexuality. As your dog is re-acting to a outside stimulant I can see no reason to have him 'done', particularly as you are quite opposed to it. The bitch's season will end and if you are no exposing him to the scent he should be fine.
When I have another cocker it will be male and I have no plans to have him castrated ( unless there are medical or other problems).