I've been through this. There is a lot going on in your home right now. You are emotional I'm sure with your wedding just a few days away. Your son can sense this, so maybe right now, Daddy is more fun, less stress. But that's ok.......He will go through these spells for along time so be prepared. And maybe not liking the step dad, no matter how great he is.
My question is this, could Dad have said something about Jason taking over for him or anything like that? That could make a difference as well.
How Dylon is with Jason will depend a lot on his father. Would your son's Daddy have an issue with Dylon calling Jason Daddy? Sometimes they slip up, and you don't want to make a huge issue out of it. If the X doesn't mind, then let Dylon call him whatever he wants......Jason, Daddy, or whatever........we didn't push for our kids to call us Mommy or Daddy.....we told them who was who. My X has been remarried many times, but, I never got mad if they called one of his wives Mom, I figured they would slip up sometimes anyway, and if they didn't know who Mom really was, that was my fault. Plus when I grew up, I called many of my good friends Moms...........Mom and they called my Mom - Mom too.
If your X does mind, then you need to come up with something easy for the son, like Jas or whatever is easy for him to say.
Congratulations on getting remarried, and I'm sure everything will be fine. Just don't confuse Dylon, be straight with him and everything will be great. Good Luck and take care.