It is not our place to judge their hearts, particularly since we have no way of knowing their reasons or intentions. To you it may not be a good reason, but to another person it may very well be a perfectly good reason. For a cradel Catholic to miss Communion week after week, it can be quite devastating. That is why so many people ignore Church teaching and go on up with everyone else and not even think about it. This happens more often than the ones who leave early.
From my understanding, the Church has never set an amount of time that someone must be at the Mass in order to fulfill their obligation. I doubt they ever will because they realize life happens and we are in no way able to control how life plays out for us. It is so wrong of people being vilified and compared to Judas for coping as best they can in the circumstances in which they live. My mom and dad grew up during the depression. They told us that often times people would arrive at Mass late and leave before Communion. Many rarely received Communion. Many heard the Gospel and homily and that is it. But the lines at the Confessional were long–much longer than they are today. People didn’t leave the Church in the numbers they are leaving today either. The people who made it in time and stayed for the entire Mass felt they were lucky that they were in better circumstances. They knew they were blessed to have that opportunity. That is why my parents taught us we should always arrive early and stay until the end of the last hymn, but never assume that those who didn’t were bad Catholics or poorly catechized Catholics. There may be a day when we too will only be able to go for a few minutes at a time for reasons no one else knows or understands. Everytime I make it to Mass I am happy to be there. Thankfully I have only had to leave early twice. I’ve been late more often, probably about 20 times. Having a deployed husband and 7 kids to get ready and out the door is quite difficult at times. Before I starting homeschooling, my kids missed the bus quite regularly, so it isn’t just getting them ready for Mass that was hard. I have had to bring my kids out due to noise and behavior, so if that is considered leaving early than I have lost count of the true number of times over the last 20 yrs I’ve left early. I would hope that the OP decides to explain to her son that it is unkind to keep count of the number of people arriving late or leaving early, and explain to him that no matter where they sit they are still at Mass and the focus should still be on the altar. No one would ever notice what the people around them are doing if they were honestly focussed on what they should be focussed on.