I tend to be informal and I love talking about myself so I will always respond. I don't quite understand the relationship here...a professional colleague? I would respond more or less honestly and assume the person really does want to know...bc I have sent many an email where I haven't inquired or commented as to t he recipient's wellbeing when I wasn't particularly concerned with knowing.
I would say, "I'm great but super busy finishing this grant...I'm doing well today; recovering from a bad cold last week...I'm am thrilled, looking forward to the weekend...I'm doing well and I hope you are, too."
Does the person have some specific reason to be concerned about your wellbeing? Had you mentioned or alluded to something the last time you spoke? If so, it would be rude not to respond IMO.
3b (with 3c tendencies) on modified CG