Despite the importance of your love and support, it isn't always enough. Many people who have been traumatized need professional help. But bringing it up can be touchy. Think about how you'd feel if someone suggested that you needed therapy.
Wait for the right time to raise your concerns. Don't bring it up when you're arguing or in the middle of a crisis. Also be careful with your language. Avoid anything that implies that he or she is crazy. Frame it in a positive, practical light: treatment is a way to learn new skills that can be used to handle a wide variety of PTSD-related challenges.