I'm not even sure if this claim is true, but...
This source states that
Caffeine gives both types of sperm a boost, but the Y-sperm would get a little more of a boost.
This article reinforces the quote. In fact, this "tip" can be seen all over the internet to preferentially conceive a male rather than a female.
My question is how. What is happening at the molecular to increase the overall mobility of sperm? It clearly wouldn't work as a neurotransmitter (as it "normally" works).