I sympathize. My XW's chronic lateness is something my kids have had to deal with. There's a lot of sitting around and waiting for them when it's her turn to take them to dinner. S12 has spent an hour looking over his shoulder at a football game, wondering where his mom is.
H, Even though I don't really post here much anymore I do try to keep up with the people like you that went through it during the same time. I am dealing with this exact thing right now, a little different as the activities she does show up for she is on time! lol My S14 mentioned to me the other day that mom misses all my games, does she even care what I do anymore?? I felt bad for him as I saw how disappointed he was, I told that him I'm there to watch along with grandma and grandpa and I will always be there for him! I also said to him that if he wants his mom to go then talk to her, his response was...... I don't care about her anymore so I don't need to talk to her! And then I didn't know how to respond to that.
So, reassure your S12 that you will always be there and encourage him to talk to his mom if he wants her there. Focus on you always being there, I know it may not feel like it from time to time but the kids will know who put in the work for them! You're doing a great job!! Have a great weekend