Answer: Pain with penetration is not normal. If there is pain, it is not unusual for erection to be discouraged and for arousal to be diminished. You don't mention your boyfriend's age, but I recommend he see a urologist (or other M.D.) to have the pain evaluated. In the meantime, don't put effort into "keeping him turned on." Once the pain is eliminated, the "cure" is to find ways relax and enjoy one another.

Intimacy Q & A: Erectile Difficulties