My fiance wants to leave to go to his dads for a couple of months, to be around him more. He says he misses him. I get it. He has barely been around his dad most of his life. But for a couple of months?

I cant go btw because of birthdays and life. So don't say I could go with.... But what do i say to him...? I dont want him to leave for that long... I cant handle that mentally.. Idk why it has to be that long and why we cant go together but a few times a year.... Help.... Idk what to do..