Welcome! Glad you are asking questions and welcoming feedback.
I don't consider he decided to leave the religion, but is simply taking the path of least resistance once his parents and congregation put pressure on him. He only decided to hide from his parents and congregation, his relationship with his 'worldly' girlfriend.
He did lie to you already as he wasn't upfront with you about who he really is.
Even as he is now no longer practising, unless he recognises for himself the JW religion is not "The Truth' and is a cult, he will likely continue to view JWs as god's only true religion, and have high probability he will start feeling guilty and want to return.
Even after being away for some time, even decades, he may decide to return when there is a major life event such as the death of a close family member, a major local or world event (ie: 911) or birth of a child. These events cause a person to contemplate the purpose of life and then often result in wanting to regain practising what they are familiar with and told is "The Truth", so as to save themselves and their spouse and children.
If he does return, he may attempt to convert you and failing that, demand any children you may have together, to attend with him.
Although married to a JW, you will not be the most important relationship to him as Jehovah and the organization will always take precedence over you.
If you search this site's archives, you will come across numerous postings on people asking similar questions as you Even as they asked for advice, many of the prior posters didn't appreciate the advice offered.