My 2 year old hurt his thumb, the nail is looking purple-ish underneath and the nail looks almost like it will fall off. should i take him to a doctor to get it seen to, or just leave it?

Basics: What you describe is usually a buildup of blood as a bruise under the nail. If treated by a doctor within the first 24-36 hrs (a tiny hole drilled in the nail) the pressure is released & the nail is usually saved. If not that pressure can cause the nail to eventually shed over days/weeks & it may injure the nail bed causing a deformed nail.