Yep, but maybe he thought he had an escape clause with non-marital partners, well that is until khadija found out.
Was the concubines child the only child of his that died - who knows how this child really died ?
No, he lost three sons that are known, according to Muslim historians. Ibrahim who was the son of Marya al Qubtiya and Abul Qasem and Abdullah who were the sons of Khadijah. Ibrahim is said to have died from a fever. All the sons died in early childhood. Frankly, it's not like it was highly unusual for children to die back then in early childhood. I'm sure those of us with family in the Middle East or South Asia have mothers or grandmothers who lost one or two children in early childhood too.
You know, though, I bet there were other female children who died in their early years and that some of the other wives had miscarriages and this was never recorded... that they weren't important enough to mention. I do believe I read once years ago that Aisha suffered some miscarriages and was believed to be infertile, but I couldn't tell you where I read that.