Translation:Once I love someone, I will make him happy.
Shouldn't it be I will make that person happy - we don't know if the one we love is a man or a woman in the first clause. i.e. Một khi tôi yêu một ai đó, tôi sẽ làm ai đó hạnh phúc.
We do know the gender of that "someone" because it's revealed in the second clause. This is a very specific sentence. Very likely to simply test our understanding. This sentence would come from someone who prefers men.
may I just change your sentence to sound a little more natural, I personally would say: một khi tôi yêu một ai đó, tôi sẽ làm người đó hạnh phúc.