It’s still a little startling to get tugged into a hug without warning or obvious explanation. (He still hasn’t decided what he thinks her motive is, but she seemed nice -- not the sort who would pull him into something if he told her to stop, so she should be safe.)
She’s also kind of soft, and her touch is nice. A bit too much of a dog, having his hair petted is enough for the metaphorical tail to wag. Cumulatively it’s enough for him to relax and let her hug him (though something about this seems... embarrassing?) He returns the hug, sort of, tentatively, with only one arm. It seems the appropriate kind of response to have. “Can I ask... why?”