Is a boy gay if he wears his pants down low?

Yes and no. Historically, it was common practice in prisons to wear your pants quite low to tell other inmates that you are gay and willing to have anal intercourse. This gave other prisoners the "go ahead" sign that you were willing to engage in sexual activities with them.

However, this has changed since then. It is now common for many males, young teenagers in particular, to wear their pants quite low.