You two are dating, you shouldn't be scared to bring things up. Just be sensitive to the topic since it obviously will be a soft spot for him. Why don't you text back and just ask, 'how so will you be sad?'His mum might not be dead but there are a number of reasons to what could make him feel that way.
Offer to see him or spend some time with him. He'll appreciate you trying to learn more about him.
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Aimforthehead and I are cooler than cookies and creme.
The adventures of Chickadee and Chicklet
Ex-stalker of Gazzy