I also used to lie for no apparent reason. If you go to the top of this forum and click on the "sticky" titled "13 common characteristics of ACoAs" you'll find it's not at all uncommon.
I could not tell you why I used to lie. I knew I was doing it. I didn't like it. I felt like a total sham. I couldn't stop myself. I can't, to this day, tell you why I did it. I stopped when someone I cared very much for told me I needed help. Just as you told your bf he needed help.
You did exactly the right thing - you did not confront in an angry fashion, you confronted with love. It sounds like you weren't judgmental, but that you were worried about your future together. He sounds like he may be ready to start confronting his own demons, but needed that little extra nudge to get him there.
His path, if he sticks with it, will be a long one. I will warn you that if he does stick with it, you may find that he grows into a different person. I am certainly a far different person than I was before I started therapy. I would not be compatible with the person who originally nudged me into therapy now. I'm not saying that this will definitely happen to you, just that the possibility exists, and I want to give you warning about it.
Even if he does become such a different person than the one you know now, he will most likely thank you in his heart and mind for the rest of his life for starting him down the path to recovery.
The best way to not have this emotional separation is to listen to him. Ask him if he wants to talk about his sessions, but don't push him. He'll need time to roll things around in his head for a while, but he may also need to let some of these pent up things out. The life of an ACoA as a child is one big lie. Parents lie about what they will or will not do, they lie about how much they do or do not drink, they tell their children to lie about the functionality of the family. Is it any wonder he lies? It's what he was taught and a behavior he had modeled. And the low self esteem doesn't help - it only encourages the lying.
Recovery is a long and perpetually ongoing process. He will need support, although there may come a time when you can't provide that for him. He may also find that he just needs to be utterly alone for a long period of time (I became a social hermit for about 3 years). If any of this comes to pass, know now that it will not be because of you - it will be a result of him wrestling with his demons.
I wish you both much luck and peace.