Depending upon how old he is and how long he's been a Christian.... has anything that he believed in before changed? Has he changed his mind from his early walk until now?
The point of the matter is that there are different theological interpretations of scripture. Nobody can be certain in non-essential areas of the church. Doctrinal issues will abound and none of us can be 100% certain.
These things are simply what we believe to be true. And I for one defend what I believe to be true unless I truly have not taken a side on an issue. If I believe something is a certain way then I will defend it until someone gives me reason to pause or to change.
The essentials -- Jesus is God, there is the Father, Son and Holy Spirit, Jesus is eternal and is not a created being, etc. -- are simply non-negotiable. But even here we have disagreements about how to interpret certain things (like the person of Christ).
Most things in OSAS can be interpreted in the lost salvation thought anyway: "He lost his salvation" vs. "He was never saved in the first place.". And many others.
The difference to me is how I perceive God. In a lost salvation thought God can be seen vindictive, uncaring, not in control. How could He let a child become lost again? But in OSAS He is in control. He will keep me and it is not up to my "works."
So, if your step son changed his mind on ANYTHING, then he simply cannot be certain that he will NEVER change his mind on OSAS (I won't, but he might

).