5 year old reads, writes, adds, multiplies, subtracts, and comprehends conservatively as a 2nd grader. When he speaks, his thoughts are jumbled and frankly incomprehensible. Smyptomatic of anything?

Expressive Language. Could be a form of an expressive language disorder where expressive speech is an issue.Needs an evaluation. Call your peds.
Normal. Children all grow at different rates. Your child sounds like he is very bright. Children commonly have yet to grasp the ability to filter their thoughts and words. As such, his fast thought process gets ahead of his mouths ability to speak. Asking and encouraging him to express his needs and thoughts in full sentences is not unreasonable. Be patient and let him speak.