I can't address Pulp Alley, and haven't played IHMN but…
After playing ITEN and reading through the IHMN rulebook, if your son can handle SBH, he can handle IHMN. I'm looking forward to playing IHMN when my RAFM Airship Pirates KS delivers. Pulp Alley is also a very well regarded game though.
Really though, there's no reason to limit yourself to one. Neither rulebook is particularly expensive and you can likely use the same figs in both games nearly interchangeably.
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