Ah @StugglingtocopeinEdinburgh, that is such good news that your DS has empathy for both his sister and animals. Which also suggests to me that some outside behaviour from other people may have caused his apparent lack of empathy towards certain people.
So I suggest that if you ever get any quality time to yourself could you write down two lists just for you - using a pen and paper is usually more effective than typing on your phone or computer - of people that your DS shows anger and lack of any empathy with, and those that he does show empathy with. Maybe start with the most important people in his life ie his family. If you could label the two lists as something like a and aa, in an attempt to not add too emotional a title to each list. eg 'good' and 'bad' are not very useful terms to use on this type of occassion.
Under one column you would put the name and relationship of someone that your DS has shown empathy for (eg Sally sister) - or types of people (eg boy or girl, man or woman) - and under the other column someone who he has shown some anger AND lack of empathy for ( eg Jason Daddy). If you know which column you would come under, then add your name and relationship. If your DS has anymore siblings then add their names etc too. Then Grandparents if relevant, cousins and Aunts and Uncles, children in his class that you know about, teachers etc. You might find it helpful to have a third column for those that you don't know if they would fit into either of the original columns.
When you have written down all the people you think it might be helpful to look at, can you then look for any patterns, some might be more obvious than others, eg are there mainly males or females, or roughly the same amount, are there mainly children or adults. If any category shows a marked increase in people your DS is more aggressive and/or uncaring with, can you see a pattern that puts them into a sub category of eg. only female children over the age of about 8 or 9 years old, or only male middle-aged adults? I am not going to try to tell you how to interpret any patterns you may or may not see, after all, it could be completely random, and even if there is a noticeable pattern please remember that your dear 5 year old might not be aware that he has such a pattern, or even why. So please don't confront your DS - I'm sure you are sensible enough OP to not do that, but I still felt that I should say it, as you are probably in a very emotional, and maybe confused state at the moment - or anyone, including your husband, your parents, your friends etc. who are personally involved. I would suggest that you need to see your DS's GP, who is hopefully yours as well, and discuss with them who they think would be the best professional type of mental health practioner for your DS to be referred to.
My personal instinct would be a child psychiatrist, one that has many great testimonials. Unfortunately, to find someone good enough, and within a short time frame, it might be necessary to go to a private - or joint with the NHS - practitioner, as I truly believe that the sooner your DS can get help, the more positive the outcome will hopefully be. Bless you OP, I hope that life can become a lot easier for you, your DS. and anyone else who loves and cares for you both 🩷