I agree with flowerchild.
Furthermore, you are putting yourself in direct competition with this teenager for his father. NOT a good idea, you will not (or at least should not) win that battle. As much as you would no doubt choose your kids over your partner if needed, you cannot expect him to do anything else either. It is irrelevant that he's kid is older.
On the topic of older kids, I think it is particularly important in those situations that the actual parent do the parenting - not the step parent. Because teenagers (particularly an 18yr) is not going to veiw you as having any authority, and will only end up resenting you.