My Son Is 3 1/2. He's Got Sniffles, But Nothing

My son is 3 1/2. He's got sniffles, but nothing else. The past few days I've noticed blood in his nose, and today when he sneezed, blood came with it from his nose. I think it's probable that the weather changes (it's been fluctuating from 30 degrees to 50 degrees the past week) along with a dry house (wood floors suck up all the moisture) are causing the problem. When I've turned on the vaporizer in his room, it seems to help. Any cause for concern?