Thank you for this additional information and no worries on the delayed response!
I do think castration was sensible. It can take several months for testosterone levels to fully deplete so it may be some time still until you reap the full rewards. Of course, removing the testosterone is only one piece of the puzzle and training / managing the environment will also be needed.
A complicating factor for Alfie is the recent orthopaedic surgery, which may be making him feel vulnerable. It can take quite some time for dogs to feel 100% and he may still have a slight niggle which is making him more defensive around other dogs. It may be worth having a specialist check him, just to ensure his knee is fully healed and there is no sign of e.g. early arthritis which may benefit from intervention such as anti-inflammatories, physio, hydrotherapy or pain relief. Joint supplements would also be advised if not already something he is taking.
As the tennis ball was a factor, he may well be a dog who guards resources; which is not uncommon. Due to this, when around new or unknown dogs we should have a blanket 'no food, no toys' rule.
I agree it is best to only walk him on lead for now if there are other dogs around; both for him and for them.
Be sure to hold his attention while walking him on lead; this usually means positive reinforcement in the form of praise and treats when he listens to commands such as 'sit' 'heel', 'wait' etc.
We can also make the walks more fun with scenting exercises i.e. dropping treats for him to find and bringing him to places with lots of trees, plants etc to have a good sniff.
As mentioned, a consult from a behaviourist would be sensible as there are a few things going on with Alfie and there has been more than one incident. They will be able to give you more advice on managing him on the lead, as well as more generalised guidance.