trouble getting him hard

So me and my boyfriend have recently decided to take our relationship to the next  level, we've been together for about 6 months and we are both virgins, lately we haven't had the time to have sex but we heavy make out and everything else. Last night and tonight we had the house to our selves but he couldn't get hard, we made out, he went down on me, I tried to go down on him and nothing!! It's making him really upset because we've both been waiting a while now and I'm not sure what causes this and how to help