Feeling a bit rubbish

I'm sorry that you're going through this rough situation, particularly after having navigated a miscarriage and the emotions that comes with for both of you.
Mens mental health is often sidelined in these situations. If he is telling you he is tired and stressed, I would probably start there. He very well might be. Maybe talk to him, encourage him to talk about his experience or engage with some level of counselling. Parenthood takes a toll on men too, postnatal depression in men is very real and often triggered by watching their partner that they love go through something awful that they can't help with. Dads groups or clubs built for getting men talking about their feelings and experiences really help.

I wouldn’t automatically assume he doesn’t find you attractive. The fact it started after a miscarriage and during pregnancy makes me think there could be more going on mentally. Equally, if he’s still masturbating regularly, sometimes people get into the habit of choosing that because it’s quick, easy and low-effort compared to partnered sex, especially when stressed or tired. That doesn’t necessarily mean he’s not attracted to you, but after 14 months I’d want a more honest conversation than just ‘I’m tired.’