I have recently made a rather epic story for my players to play though. (In the sense of a long overarching story). I have been running for a few sessions but a particular player is constantly trying to do things without the group like travelling in the wilderness away from the group to check up on the people they rescued or attempting to get the villagers to build a palisade while others are busy following the plot.
I don't want to handwave what he is trying to do by saying "Okay. The guys you rescued are fine. They are making palisades now and you join the group." but he is slowing the game down and when he is doing his own thing he doesn't watch what the group is doing and then he gets confused as to why combat 'suddenly' started and explaining what is happening to him essentially ruins the tension.
I tried talking to the player and he apologized for doing it but then in the next challenge he immediately got an idea and left the group to try to apply it.
The best solution I can think of at the moment is doing mini sessions where he and whoever is ready to play at the moment will try to work on a project unrelated to the main plot though that would place a big burden on me and I fear that if I do that he might start asking for those every 2-3 days. For reference we play biweekly.
What can I do to make him stop just leaving the group or at the very least make sure he doesn't slow the group down?
As a note I know there is a question similar to mine but my problem is not that the other players cannot do things but the fact that he does one thing while the group is doing another resulting in my attention being divided and the game slowing down.