Im not a step parent (or BoM) but if it makes you feel any better I can share the experiences of my children who have aquired two younger step siblings.
They love their dad, but like most boys of similar age to your DSS have absolutely no interest in their younger step siblings or in doing any activities that the little ones want to do. This isnt them being difficult, it is just totally normal for kids of their age. Unfortunately, the desire for everyone to get along is so high in their dads new set up that they end up fed up with being constantly irriatated by the little ones and expected to entertain them and play games that they are not interested in.
Also, in a step family, when things go wrong what would be seen by me as a totally normal sibling scrap, can get blown out of all proportion. The bottom line for them is that they love going to dads but much prefer it when their step siblings are not there.
My ex takes them away skiing without their step siblings who are younger and cant ski and it wouldnt be a problem at all if he took the little ones away without them to do something age appropriate with them too. In fact they might all be happier.
Also, I have a totally different parenting style to his new wife and I'm sure this causes problems. I am far more liberal and take my children climbing, sking and on all sorts of risky adventures. For me its just what you do as I was bought up that way. Even my ex was astounded at watching his boys climb recently. But I'm sure his new partner thinks I am quite bonkers. By contrast my children think hers are babyish for their age as they dont ever do any of the stuff we take for granted.
The message, if there is one, is do what works for your family, set out the ground rules for your house, but try not to make any of the children feel less important than the others. The only time there are issues is if they feel their dad has let them down.