i guess it all depends on the exact situation and the people involved.
i do not know you both personally, but i’ll try my best.
there are certain things which the phrase ‘i love you’ is associated with.
first, are you certain you love him, or is it just infatuation, which due to the circumstances is pressuring you into saying ‘i love you’
secondly, will you be speaking with him after he moves?
unfortunately this is teenage life & not a book, so saying ‘i love you’ isn’t going to end with him moving back trying to find you 5 years later.

though, if you believe he would care to hear those words, try it. by adding that phrase with other words it may be lighter, not as serious, which may be intimidating to some. love doesn’t have to necessarily be a sexual attraction, telling him ‘i love you’ before he leaves could let him know you care for him.
i’m sorry. this is terrible.