A boy must save the magic in an alternate universe or have his own universe destroyed.

There are a few elements that if added to the logline in a concise manner would improve it.

1 What is threatening to destroy our universe. (Or the lead character’s universe) This will help us understand the overall problem and also the stakes.
2 You say ‘a boy’ but what special powers does the boy have so that we will know why it must be him that goes on this quest… (And it must be done in a word or two) When a “magically adept” boy (For instance)
3 How do the two universes connect? If the threat is in this universe why must he go into a different universe to save it?

Anyway, I believe adding these specifics will help the logline as long as they don’t add too much to the overall word count.