Since

Since
 

Could I say this?

-John failed Japanese when he was in primary 2. Since then, John decided to boost his Japanese.

The usual way is 'has decided', but this time it sounds OK to me. What do you think about the whole sentence?

The sentence sounds good, and it shows good grammar.

But do you think 'decided' and 'has decided' are both correct? If yes, what's the difference in meaning?

Yes, both are correct. Typically "has decided" indicates a stronger degree of relevance through to the present moment. However, in this context (after "since then"), one can use either with little difference in meaning.

Good answer. Thank you.