My bf can’t keep it up.

So recently my bf hasn’t been able to keep his hard. He gets hard when I give him head and I can feel how hard it is. Then when he gets up to put on a condom he loses it. Any ideas why this could be? We’ve had sex before so this wasn’t a problem until recently. He’s very fit and eats healthy, 20, and goes to the gym regularly (no steroids). Please help a girl out! It’s driving me insane and bringing down my confidence.