Catholics are also Christians remember that. The fact your son is dating a girl who goes to a Christian church is a wonderful step in the right direction. It is normal for children to wander away from the church. I grew up inside the United church and went to the Pentecostal for many years until I caught the Catholic bug. I suggest that you keep your eyes open for Catholic youth groups and fun Catholic activities that are happening inside your diocese. When you hear about them, casually tell your son about them and encourage him to bring his girlfriend along. Whatever you do, don’t push it, let him go when he is ready.
You can always keep a couple of books at home that you can lend to him such as Catholicism for Dummies, or Why do Catholics Genuflect by Al Kresta. As a mom, you can read the books yourself, and keep them in your living room for whenever your son visits. Sometimes silence and prayers are the best ways to bring your son back to the Catholic Church. Even if he doesn’t return, as long as he is going to a Christian Church that is the main thing. The Christian journey is all about developing an active relationship with Christ. They are great places to build that foundation. They helped me a great deal and their teachings gave me a good foundation that when I caught the Catholic bug, I was able to take my relationship with Christ to a much deeper level. It’s best as a mother to keep your ideas to yourself and let him work it out.
Keep praying for him. The prodigal son eventually returns home sometime. Remember at least he is going to a Christian Church which means he is walking towards God. I’d be extremely concerned if he was getting involved in a cult, or another non-christian religion. That’s where his soul would be in danger.