You are NOT causing him to have difficulties with ejaculation. Your sexual inexperience would have nothing to do with his slow or no-existant orgasm. Most standard sex positions work very reliably in terms of resulting in a man's orgasm. The sensation of being in a vagina is all that most men need. So, relax and don't worry about your role.
Things that can influence a man's orgasm speed include: recent alcohol or recreational drug consumption, some prescription medications, "performance" anxiety, and, importantly, not enough time and effort spent on foreplay, including oral sex. Perhaps slow down at the beginning. Don't rush into intercourse.
