If he put it on flight mode, put it in his pocket and never took it out while you were together, it's likely he just doesn't want interruptions to his time with you. Even then, it would be more common to put it on silent, so that he could check from time to time that he could see who had been contacting them.

When a partner puts their phone on airplane mode in a relationship context, it can signify several things ranging from healthy boundaries to potential deception. On the positive side, it may be an intentional "digital detox" to show you exclusive attention, avoiding the "phubbing" (phone snubbing) that often erodes intimacy. He might simply want to ensure quality time isn't interrupted by work emails or social media pings. However, if the behavior is secretive or only happens when certain notifications are expected, it can be a "red flag" for hiding communication from someone else (infidelity) or avoiding accountability for not responding to messages later. If the phone is put on airplane mode openly with the explanation "I want to focus on us," it is usually a sign of respect. If it is done covertly or causes him to become defensive when asked, it may indicate he has "shit to hide," as some relationship experts suggest. The only way to know for sure in your specific dynamic is to have an open, non-confrontational conversation about it.