In plain English the line is saying "The boy's mite was like the widow's mite".
This is most likely a reference to the story from the Christian Bible, specifically in the book of Mark, Chapter 12, Verse 41-44.
> He sat down opposite the treasury and observed how the crowd put money into the treasury. Many rich people put in large sums. A poor widow also came and put in two small coins worth a few cents. Calling his disciples to himself, he said to them, 'Amen, I say to you, this poor widow put in more than all the other contributors to the treasury. For they have all contributed from their surplus wealth, but she, from her poverty, has contributed all she had, her whole livelihood.
So the story is of how a poor widow gave all she had to the church and therefore was view as giving the most of all those that had donated.
Without further context, I am guessing the boy in _Don Juan_ made a similar gesture of giving all he had to something or someone.