He can't seem to get fully hard when we are trying to have sex..

Maybe he isn't ready. His mind is saying hes ready but his heart isn't. He is younger than you so that makes him more nervous. Just give him some time. And don't pressure him. Let him know you love him and you just want to spend quality time with him. And take it from there. If you watch a movie and just get him relaxed then he might loosen up a little bit!!!!! :-)

do not pressure him. that is 100 percent what is wrong . teen mom is right ...

but on that note . "teen mom" did that and now she is a teen mom

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Definitely sounds like he is getting nervous.

Don't pester him about it. That will just make it worse since he will only be able to think about the past problems when you try to have sex again.