K.F.
Yes you will miss him. Yes the other kids may be upset. Yes you will miss him but don't rob him of the time he will spend with his BaPa.
Do something extra special with the other kids while he is gone.
Your other children are learning that life is sometimes not fair but that doesn't mean you can't just move on to the next thing and enjoy what you have. How are you going to teach the others how to handle life's disappointments?
Don't worry about the diet. When he comes back home get him loads of fruit and vegetables. He will be fine and so will you all. Help BaPa to understand that next year he may have to take 2 kids or agree to do something special with the others too.