im a man and from my view

1. your bf should exercise more for better physical maintain and can release stress somemore.

2. maybe you can try wear some sexy lingerie or cosplay to get him stimulate more.

3. try not to have sex too much, once in a few days is ok unless your bf is hardcore.

4. when having sex try bed scream so that your bf can feel more proud of himself and excited, this can help his erection longer since he feel so excite and stimulate.

anyway, try to make your bf feel better before, during and after sex so that he wont feel bad of his skills and feel more confident about his skills