My DS had a hyrdocele that my ped noticed and referred us to a urologist. We watched and waited until he was over 2. The urologist said that if they resolve on their own, they will do it by around 2. The ped continued to keep an eye on it and a few weeks ago (he's almost 2 1/2) DS had the surgery (which went very very well.) I would bring it to the ped's attention and keep an eye on it. There's a really good chance it will resolve on it's own, if it doesn't by the time he's two, you can take it from there.
I would touch base with a doctor just to rule out any other issues and to learn about signs of a hernia, etc (which is what your main concern is if your DS has a hydrocele.)