Ask HN: Non-technical employees don't get along with engineer

I'm the CTO of a small startup with 9 full time employees. We have 2.5 engineers - Me, a senior SDE hired a year ago, and a new UX designer who doubles up as a part time FrontEnd Dev. 4 non-technical employees have logged complaints that my full-stack developer is curt/cold/condescending although the designer and I get along with him just fine. I've drawn a pretty hard line in defending him thus far, but have been taking more and more fire from the CEO. I've tried many process tweaks, 1:1 feedback, etc. Do you guys have any ideas to de-escalate or help the team get along better with each other?