I'm wondering if your son might suffer from GAD (Generalized Anxiety Disorder). This disorder often increases at about 8 years of age. I might suggest you google words/phrases as "GAD and children" or "anxiety behaviors in children" or "anxiety disorders and children" or similar words/phrases. If you feel that your son could be suffering from anxiety, there is lots and lots of help available. But first, see your family doctor. If he/she is unable to help you, then ask for a referral to a medical mental health specialist with experience in anxiety disorders as a child neurologist or child psychiatrist or pediatric behaviouralist, etc. A "therapist" can help; but since anxiety is an emotional, mental and physical disorder, a medical doctor will be required. But note that anxiety is a very common issue and highly treatable.
If anxiety is the issue, keep in mind that research has proven anxiety is a genetic trait and not caused by life experiences (although life experiences can exacerbate the problem). By the way, you also might wish to disucss this issue with his teacher - I suspect he also might have "fears" related to school. All the best ...
I would for sure speak to a doctor and then maybe to a therapist.
Could he have seen something that scared him when he was 5 year old, maybe on TV or the computer, you say he feared the police then would you know why had he seen a scary show has he older siblings ? It may be a good idea to speak to the doctor about his fears .Have you asked him why he feels afraid to go swimming when previously he has been okay ?