I wouldn't be overly concerned about him going to the memorial, but you should have a conversation about it with him.
Kingdom Halls fill during the memorial with studies, unbelieving adult kids of JWs, disfellowshipped, inactive JWs ...
Some might still believe, some are simply trying to not burn that last bridge with family.
Some who are disfellowshipped work to get reinstated to regain some family association and then fade away.
If you are serious about a future with this person, you need to understand where his thoughts are.