Q: if my dad cosigns can he b taken off after 6 months?
A: Only if you refinance into your name alone or you can "assume" the loan yourself - both of which require you to individually qualify for the mortgage on your own (or you can get another family member to co-sign).
What is the reason your father wants off the loan in 6 months? Is it because he is going to try for some financing on his own later on, and your mortgage payment would prohibit him from doing so? If that is the reason, then all that would be needed is proof that you are making the payments from an account in your name that your father is not also on - then his mortgage lender will exclude the payments because they are being paid by you, and not him.
If there is another reason, please feel free to elaborate.