There's a time and a place for everything. The place, generally speaking, for practicing aikido is in the dojo. The primary reason for this is safety. The dojo environment should be a structured, orderly, well maintained place to practice. This is mostly for safety.
I think that if your step-son has a sincere interest in martial arts, he should take some classes from a qualified instructor.
Also, it generally takes longer to become proficient in aikido than in many other martial arts. Your step-son may want to take this into consideration and be a little more respectful of you while you go through the learning process. Maybe show him what you're learning, but don't try spontanious applications at this time.
By fooling around in the home, sooner or later something is going to get broken and someone is going to get hurt. There's just too many sharp edges around and too many variables.
Stalking games can improve awareness, and can be followed up with a tap on the shoulder, not an actual attack. But even this could be hazardous. What if, for example, you think your step-son is not at home and somone sneeks up on you. You may throw them and hurt them, and when the lights come on, it may be your step-son!
I'm speaking here from year's of experience in "playing around" with aikido in a wide range of environments such as bars, homes and outside, and it often ends up being unsafe, with someone getting injured, especiallly if there's some drinking involved. This applies to situations where both people know some aikido or where only one of them does.
Anyway — good luck!