I wouldn’t say it’s a grave matter, but it could be come that. If you let it slide into hardened sanctimony that those who don’t meet your standards are not “real Catholics” like you, I think it could very well become grave. It wouldn’t hurt to confess it for good measure.
But it’s really hard to observe whether someone is a practicing Catholic. Being Catholic is not about ticking all the boxes on a checklist, but about the relationship we have with Jesus Christ – first and foremost in the Eucharist.
Time after time, Jesus took in the sinner… tax collectors like Matthew and Zaccheus, “sinful women” like in Luke 7 and in John 4 (whom Jesus makes an evangelist to her people), the adulteress in John 8. He was routinely criticized for it by Pharisees, who found his feasting with low-lifes of his day reprehensible. Even in the lead-up to his passion, on his last visit to Jerusalem, he stayed in Bethany among lepers, whom Kosher law called unclean, and from there entered Jerusalem as king.
Pope Francis’ article in America Magazine speaks to this very issue, IMHO:
“I see the church as a field hospital after battle. It is useless to ask a seriously injured person if he has high cholesterol and about the level of his blood sugars! You have to heal his wounds. Then we can talk about everything else. Heal the wounds, heal the wounds… And you have to start from the ground up.”
It sounds like you have an opportunity to be part of the New Evangelism!
Edit after I read your later reply: To me, with this man you’re dating, you have the opportunity to be honest about how important your faith is to you. If he’s “the one,” the Holy Spirit will move in him. I wouldn’t suggest being judgmental about him being bad and you being so holy, but instead I would suggest trying to tell him what your faith means to you. If it’s what you feel, you can tell him that you really like him, and that you don’t like for him to be cut off from what is such an important part of your life. Maybe start by asking him to come to mass with you one time… not for a lifetime. When you’re first dating, I always say that the purpose of one date is to see if there’s reason for another date. Maybe with him, if he would come to a single mass with you, he might come to a second mass with you if it goes well. To modify an oft-used quip of Al Kresta’s “Build the Church, bless your relationship.”