Potty training

My son is 16 months old, and he was a preemie. He was 5 weeks early so he’s a little behind on some things. In my heart I feel like he’s really not ready to be potty trained yet. But everyone, including my husband, says I need to start. So I did, and he doesn’t seem at all interested in it. When he does pee on the toilet I try getting really excited and he looks at me like “what did I do???” My sister in law says I need to stop babying him and let him grow up. We have another baby coming in September so my husband, and mother in law are really pushing for the potty training. I just don’t think he’s ready so I’m not sure what to do. I’ve been doing it, but like I said he doesn’t seem at all interested in it. Should I continue or wait?