My son would like to get into this game. Not for organized play or anything, just casual.
Aside from the core set, what should I buy? What would make sense for a great starting set of expansions?
Honestly, just start with the core set. I've been playing core set only matches with some friends at work for a few weeks, and we're having a great time. The variety and balance of a single core set (12 possible matchups) is HUGE, and there's also plenty of room for deckbuilding (as long as you're only building one corp deck and one runner deck at any given time).
The only thing the core set is lacking is agenda variety--each corp only has 5-6 points of in-faction agendas, so running every neutral agenda is absolutely necessary.
"What Lies Ahead" (the first ever data pack) remedies this by giving each corp three copies of an additional agenda, but that's only if you want to really start tinkering with the corp decks.