Trying to have a baby

So my fiancé & I are trying 2 get pregnant only one problem he can't cum :( ... When we 1st started 2 have sex he would pull out and cum did come out but later he said he would be cumming and nothing would come out an it's the same now he says he feels like he's cumming and he says he has finished but nothing comes out and I know it makes him sad cuz we want a baby but I don't what's wrong also I read it could be cu he underweight and that can be an issue & I know he's under weight so what vitamins should he take and also he's been stressed since his brother got locked up