You can be supportive of your boyfriend without hiding the fact that the responsibility is on him. Idk about you but if someone were faking their support for me it would bother me if I knew that, and I'd much rather think my friend, gf, or other important person in my life be completely honest with me.

Mind you I am not saying that you should be cold and uncaring about how he's enduring pain for it but put that where it belongs. The pain caused by him making mistakes. it's some of the best pain we should endure and welcome, because it forces us to learn from our mistakes. If you act like you're on his side feeling like his pain is anything but consequences of bad choice, he won't get anything good from the experience itself.

Be supportive of him but don't coddle or be fake.