For an assessment, I'd recommend a developmental pediatrician or a neuropsychologist. There are others who can do language testing, but those two types of doctors can do the most thorough testing and help point you to where you need to go next for help.
My DS9 uses words oddly as well sometimes, and has a dx of PDD-NOS. (My favorite of his is 'verses'. I think he picked it up from a video game, but he uses it in a dozen different contexts where it doesn't quite fit, lol. "Kayden versed me today on the monkey bars, but I won.")
