What do I do?
What do I get for him?
Should I write him a letter before he leaves

Sure,a letter might be nice.

Or perhaps make up a box of things,example,letters,pictures of you and him together,his family,friends.

You could also get a notebook and ask them to write something special for him.

Maybe a cd of his favourite songs.

Farewell sex will make him miss you more.

I disagree with this advice,we don't know what age the op is.

I recently left my girl for a year. She gave a really sweet good bye box to remember her by, covering the 5 senses. It was really sweet. Now four months isn't so bad and depending on how far away he his it will fly by. Just please please please don't be like my girl who now after 6 months into my one year of being away has given up on us and broke up with me.

But yea the 5 senses box was really sweet and a cool idea.