he wants to have a baby???????

I have been dating this guy for awhile now, I am 17, he just turned 19, he lives with me ( my mom is completely okay with him, she is the one who suggested he moves in, we've literally been together since i was 14 and he was 15 ) well recently hes been talking A LOT of babies and pregnancy and asked me if id be willing to start trying now, dont get me wrong I LOVE KIDS, i have my 1 year old nephew all the time, i love that child but i dont think im anywhere near ready to have one of my own. im only in 12th grade! i dont know what to say or do. i was completely in shock, i havent really spoke to him since he asked ( it was last night, i just looked at him and then decided to go to bed and he went to work in the morning, i went to school so we havent spoke much )