How to tell if he's masking?

I'd like to help by presenting a few different ways to perceive autism and this should help you assess better if your step-son feels safe around you.  

Masking. Jung suggested that for most non-autistics, a mask was part of social fluidity. Almost everyone masks. It's learned very early on from language and part of this automatic 'tapping into' and engaging with a social collective, literally like a bit of improv / theatre. This is part of maturing. However, due to a difference in how we're wired (and also motivated), Autistics use language different and so don't catch these transmitted 'codes' which teach Jungian masking. The Autistic "mask' actually translates to a degree of Survival Mode. 

Autism is a difference, rather than a disability. But we can be disabled by social expectations, projection and bias. That doesn't mean someone can't have an added disability. But it's good to think about the other on a scale of time. If they lived in a time before books, for example, would this person still be dyslexic? Or would their other abilities be heightened and better suited/recognised. There's 2 sides to every coin. 

The Jungian Mask creates a collective glue by inhibiting closeness. No one is overwhelmed and therefore, it's easy to "run with the pack" In fact, when no one is too intimate or too close, this drives the enjoyment of belonging and a desire for relationship. Everyone feels respected.  However, being autistic means everything I experience feels too intimate and this shapes my bias. Everything is a bit too real, too much, too intense. And this can include the deeper things of an other, only, when young, without the insight and vocabulary to name or identify the tiny universe every other human is! Everyone wants to be accepted. No one wants to be rejected. But we might not feel we connect with others and not be driven to 'belong' or we might feel too close a connexion and after an hour believe we're best friends but also need a few weeks alone.

I had to learn that not everyone experienced life this way and didn't until my mid 30s. A few theories on this difference discuss a lack of biological filtering and inhibition (GABA). Our Salience Network is sensing life around as 'too real'. We're not filtering out incoming signals the same as non-autistic peers. My observations might be closer to reality by default. This can be exhausting. And it's also easy to 'feel' unprotected and overwhelmed. In a healthy environment, I might be helped to use this 'curse' as a gift and become proficient with my senses. Like identifying 100% chocolate by region or match a frequency to its position on a piano, and so forth. At no point am I able to dull my senses (internal and external), we just end up tinnitus without proper ear protection, for instance. 

There's a theory that we're just a different design of human, and learning to identify and understand our environment, whether physical or social, just requires a bit of physics and sociology. Simple right?

With a family, the best way to help your autistic children understand 'boundaries' is by first treating them with the kind of respect you might give a business client. Given that we don't mirror the full social expression, just the surface stuff, it becomes important to Lead by Example. If you want kids to knock on your bedroom door, always knock on theirs. If you want them to ask to help in the kitchen without being told, always ask them how you can help them. Default to grounding principles, and help them mature through conscious detailed awareness of things. Help them make informed decisions by informing them on all choices. Autistic children only break social rules for Lack of Knowledge. You also won't need boundaries if children feel respected and protected. 

Unless we experience continual cruelty, it will always be an Absent Insight which needs spelling out: the underlying principle and ethic. This may mean rethinking something which isn't or doesn't actually make sense. We also don't understand social hierarchy. All adults are still that child they were born as in their core. That's the part which resonates with me. However, there have been mentors and teachers I've come to deeply respect but because of how they treated me: with respect, kindness and openness. 

As for Alexithymia, I can't tell you how I feel, either.' Feelings' are always a mixture of chaos and when causing intense emotions, sever my ability to access words. Most of my childhood was either lost in the moment and content or a great deal of confusion and deep frustration. 

If he is masking well, you will not be able to tell. Masking in and of itself is not necessarily bad for autistic mental health. Masking is only deleterious when it causes distress. Some masking, mine for example, can be tolerated well by the masker. What can happen, is that there is too little downtime - usually by being alone - when there is no need for the autistic person to mask, and that can be exhausting and lead to meltdowns and shutdowns. I would allow your step-son plenty of time to be on his own, if he wants to be, to recharge his 'social batteries'. Many autistics have alexithymia, which makes it difficult to recognise and/or express mood. Asking him how he feels may not lead to many informative answers, because he may genuinely not know how he feels.. This is another of the reasons for meltdowns or shutdowns, because an autistic person may not recognise a negative mood until it becomes too acute to be coped with, resulting in an emotional overwhelm.