There’s good evidence that testosterone drops when a guy becomes a dad, and the drop may begin as soon as he learns his spouse is pregnant. Here’s an article about it. It’s very roughly a 240 ng/dL drop based on studies.
But you’re probably overeating a bit, too, and under more stress (even if it’s positive stress).
There does appear to be something to Couvade syndrome, but it’s not an official medical diagnosis.