In my opinion, you're in a tough spot Sugar01.
We all have different idea/ls on caregiving. It is tricky when new love is introduced to the children. What's best for the children is a solid, strong and devoted relationship. It is also best that you are both on the same page and agree upon what is appropriate to introduce the child/ren to and when.
It is nice that you want to develop a friendly relationship with your bf's son. But you and your bf need to be completely honest with one another about expectations and desires. It is key that you two are in agreement about how this should be handled ~ otherwise, you've got a lot of resentment ready to build between you.
Guys typically aren't real open about these kinds of topics, unfortunately. But it is important that you both understand one another's intentions ~ so you can both make educated decisions on whether or not this relationship is going to work. Keep in mind that some give and take is going to have to be done on both sides...but it neither of you are comfortable with what is considered necessary by the other, resentment is right around the corner ~ which can build a huge wedge between you!
I wish you the best. Hopefully, you and your fiancee will be able to come to acceptable terms for both of you and build a healthier relationship. Best wishes to you!

to Psych Central btw!