Later he learns how to play the guitar and drums.
if there is how, shouldn't it be: plus tard il apprend a comment jouer...?
"learn how to" is an English construction. For a change, the French is simpler: "apprendre à".
However, pls note that "apprendre comment + infinitive" also exists.
Is ' ensuite ' an acceptable alternative to ' plus tard' ?
Yes it is.
are both "de"'s absolutely required? (i.e. does de la guitare et la batterie not make sense? Is it not commutative?
no,sorry, you have to repeat "de"
I'm not getting why "de" is warranted here. He is learning the instrument, not some of the instrument.
It's how 'jouer' works, 'jouer de' for instruments, 'jouer à' for sports or games.
et des tambours ??