I don't know much about TPRO either but DD5 took it in the fall. She's subject accelerated and reading is her strength. She had high scores in 4 of the 5 areas but her vocabulary score was much lower. I don't think this is a true weakness for her but was going to check on it at the teacher-parent conference.
When DD8 was in 1st grade, I found she had more trouble with the lower level AR tests than the higher level. She didn't care if Clifford chased a ball, a cat or a mouse and would miss those questions. Give her
Pippi Longstocking which engaged her and she'd get all the questions right. I would suspect a similar thing with your son.
P.S. Here's a link to sample items so you can see what types of questions they ask:
https://mytpro.net/public/grade1