My thoughts, sometimes you have to do it for yourself.
HR is not the answer for every single issue.
As an HR professional, my coaching is drawing your own boundaries if you can first. I only give my extra time to those that appreciate me and I appreciate them or truly need my help. My grandfather always said, work with the willing and the appreciative.
You no longer are "safe" or can trust him. Life is too short to spend on people like this and your time is too valuable. Also, people that appreciate you will give you the recognition you deserve even when you are not there.
In the future when he asks for help that is your opportunity to be very clear with him that you have decided you can no longer help him as in the past based on your experience recently with his condescending behavior on slack and also the lack of integrity regarding taking credit for work you did.
Does your organization have core values, highlight which ones he is not role modeling with his behavior.
Life is Sweet.