I didn't dare to ask, but as a man I can confirm that a condom can has a very negative impact on that one, particularly if you haven't used one for a while. And if doesn't fit properly. That in combination with a new relationship where you are eager to impress, or maybe rather being scared of *not* impressing - I can see that happening.

So, before you start looking for other reasons, just give it some time. Try different condoms, it's not so much the width or length, but rather the cut that can make a big difference. Lubricant helps a lot (both outside and inside of the condom), different positions and certain techniques you can do to help him, anything to relax before you start, lots of things to try and I'm sure you will get by.