My boyfriend didn't get a boner. help?

Nervousness if he's inexperienced or shy.

Sweetie, it is normal. I assume he is also young and is probably fairly inexperienced. Nervousness is the greatest factor in killing an erection. Make it a non issue. The more you both stress over it the bigger a deal it becomes and the erection won't get there.
A guys hard penis is very finicky and often unpredictable. You can't will it to be hard. Less thought given to the task is often the best effort. Don't try, just enjoy yourselves and each other.
Ever heard the saying that a Mans Penis has a Mind of It's Own? It is true. You can't think for it. It does what it wants. When you do get it hard, please do use a condom. Just know that once you start using a condom for the first time, there may be more erection issues. Good luck and be patient.

It was nothing that you did or didn't do. Most likely he was very very nervous. and if he had just had an orgasm, he was bound to stay soft for a little while.

He was most likely too nervous to be able to relax and let it happen. Pressure, stress, anxiety can all do that to guys. Next time just be calm about it, ease into sex, and keep it pretty calm and fun, not stressful. And if he has this issue again, make sure he knows you're cool, you understand, that you don't view him as less of a guy because of it. This can be a big deal for a guy, and you're reaction can make it better or much, much worse.