Seven months seems short. I am not judging harshly, just from my past experience. If he cannot tell her about you.... There seems to be an issue here . He seemed to have no trouble telling YOU his feelings for HER....
That may just be because you are more open and he knows that it it may mean more. Just a thought.
Being a secret is no fun. I have been a secret before... It is a hard road to travel. You will need to decide if it is worth the risk and reward.
If you deside to remain with him as a secret, even if just for a time, you will need to really focus on yourself. Being that person behind the scenes is rough and you can get anxious quickly and suffer emotionally because of it.
Confidence is key. If you can confidently be there when you are able and be the 'quiet mouse' when you need to be then you may be ok...
But then there is the deception issue.... But that goes along with what I said first. It may mean something that he can tell you but not her.
You know your guy, pay attention and learn him more. Weigh your options.