Erectile DF I never know when he will get wood.. PLEASE HELP!!!

It comes and goes as it pleases. He is only 25 and has only had sex one time before me. He claims it's always been like that. It makes me extremely insecure. Like is it me. Am I not doing my part. Because even if he has wood. It almost always goes limp in the middle of intercourse. He still ejaculates when it's soft. He always finishes.. can I conceive if he isn't fully penetrating me. Dose anyone else have this problem