I was in your kids shoe's once, although I was 11 when it happened.  I have a great relationship still with the whole family, step parents and all.  It also seems as though my parents and step parents are getting along somewhat these days now that they have grandkids.  It was a rough go for a while, but it all worked out the older I got.  Just be there for your boy, don't run down the ex and step family in front of him.  One other thing I'd add, but it's up to the ex, be flexible with custody.  That helped me deal with a lot of it.  I could almost come and go as I please as long as it wasn't a school night and they were able to transport me.  Good luck in your situation and I hope it all works out for the better.