My opinion is EftTC Vol. 2, due to his age. It’s not really a “grammar” book but a LA book-similar to ILL (Intermediate Language Lessons). Also, once a week, look into using GrammarLand with the free worksheets or Simply Grammar as gentle intros into grammar or Jr. Analytical Grammar (JAG) if you want some diagramming and a more workbooky approach (though I haven’t used this one, only GL and will be using SG this next year for my 11/12 yr olds prior to OMT in 7th). I used Grammarland when mine were 10 and 11.
It’s a good time to start written narrations, probably 1x a week. THis is also a good time to start Dictation. If you’re strong in grammar, you could use the Dictation and/or copywork to practice identifying parts-of-speech. That’s what I do (when I remember).
There’s another thread going right now having to do with grammar, writing, and vocab. that may be helpful to you to read along.
HTGH