I second Romantique Tp’s advice. Go for Dorico. I think it’s the obvious choice in this case. Dorico’s flexibility during note input is unrivaled already now. Your son will be able to focus on composing while Dorico produces legible notation reliably.
And yes, this is an investment in the future if there ever was one, so hands off from Sibelius!
Here’s a small anecdote that might interest you:
There’s an educational discount available and I think that the crossgrade offer from Finale is also still valid.