Player shows up inconsistently

I am a DM playing Waterdeep: Dragon Heist (soon to progress to Undermountain). There are three players, I play a generic paladin, and we play via Zoom. One player, however, is inconsistent showing up, and I am worried that he is going to miss too much of the story to progress. The characters are level 4, and we started doing Zoom at about level 2.

Since then, said player shows up to about half the sessions, and even then he only stays for part of it. That bothers me the most, as he is new to D&D and I believe he does not grasp that the game is supposed to take about 4 hours at a time, because he only stays about 1-2. He has missed out on much of the story and we often have to fill him in.

The other two players are fine playing without him, but I can tell that they too would rather have him play for the full length. Whenever I mention it he just shrugs it off and wants to keep playing. I am hoping there may be a solution to this inconsistent behavior.

[Note] This may be a duplicate but I am not sure