Well, we told him yesterday morning, snuggled up in bed before his sister had woken up.
His first reaction was no, he didn't want to, but didn't get upset really. He had a few questions, but not as much as I expected.
I actually thought we would get more once he'd gone to bed because that's where he does his processing, but nothing as yet. He's either forgotten all about it or he's turning it over in his head still.