So in a very short space of time he got his ex pregnant, then left her, met you, started trying for a baby with you, now has changed his mind? Yikes. I think he’s right to change his mind - you can’t have been together long and rushing into having kids is not smart decision, especially when he already has one on the way. Once his ex’s baby is born he’s going to be spending a lot more time with her and you may well struggle with that. You wouldn’t want to be pregnant and then realise you can’t handle that he’s got a child with another woman.
Telling you to abort is harsh, but I’m guessing he’s just overwhelmed and the though of throwing another baby into the mix has made him feel even more worried - as it should!!