Breastfeeding while pregnant

Hi all, I'm looking for some advice on breastfeeding my little boy while pregnant. I'm 21 weeks pregnant and my boy has just turned one and still breastfeeds 3/4 times a day. I've been weaning him onto a formula bottle because I'm just a bit exhausted doing both AND it'll be a challenge breastfeeding a newborn and 1.5 year old. My main question is... How do you know if your milk is drying up? My boy boobs for 20 minutes and then straight after will have a 7oz bottle... Is he just latching for comfort? I've never pumped so have no idea if anything is still even coming out, how do I know? My boobs are a bit sore but I think that's mainly the pregnancy... 🤔