Lack of desire. Help and i would be so!

Recently during sex for the first time my male friend had difficulty in keeping erection and due to which there was lack of desire during sex. At normal times there is visible erection but at the time of sex there is problem. So could you please help me out as to why this is happening and also can you suggest me proper ways to overcome this problem so we could enjoy the sex . It would be of great help and i would be so thankful to you . I want to support him as much as possible so can you provide me answers for 1. How to keep proper erection at the time of sex? 2. Any ways through which we could overcome the problem? 3. Any home therapy like massaging the area or exercise would help? 4. Is it normal to have this problem during having sex for first time? 5. Can it happen because of stress and anxiety? 6. Does Watching porn or masturbating too muchcauses this? Please help me . Iam in need of ur help