What to do.

My fiancé works 8 hour days, Sunday through Thursday, on 3rd shift so it's 11:00 p.m. to 7:00 a.m.. He is now a lead technician, but it's only temporary. He started this 3rd ship position in February. In April, he was given a raise and then he was asked to be a lead Tech because the one that was there was not doing his job and they've been trying to get my fiance in a lead take position for a long time now because he's good at what he does. Today his supervisor pulled him aside and said that he is pretty much as an idiot because they want him to go to 12 hour shifts to permanently stay a lead technician but the issue is with my fiancé's son. He gets his son every other weekend from September to June, then from June to August there's a different summer agreement. With the 12 the shifts, the schedule isn't consistent and no guarantee that he would have the weekends off when he has his son. My SO dilemma is should he go to 12s to stay a lead and keep his pay, but may have to work weekends when his son is here or be demoted and lose the pay, but have a guarantee he will have time with his son?