Is it abnormal for a 17-year-old to have trouble maintaining an erection with his girlfriend?

Not abnormal! While this is deeply troubling to young people, it is unlikely anything more than performance problem due to stress, anxiety, emotional conflict, fatigue...And the list goes on. Every adult male will admit he's had this problem at least once, and for many it is frequent. Forgetting stereotypes, and talking frankly with a sympathetic health-care provider is a great start to improving this.
Yes. Erection problems need to be addressed at any age but especially in a young person.