If you feel that you want to tell him now, maybe this is a good time.
He sounds like a nice guy, so hopefully he will be supportive. If he's not okay with it for any reason, it's probably better to know that sooner rather than later before you get more attached.
Hope it goes okay. Is there anyone else who knows about the SI that you can talk to before you tell him, so they can check you're okay after? Although, you can always come here and tell us xxxx