A freshman would conventionally be when they are 14-15, so you could consider him a freshman or a sophomore. If you are going to traditional public school, you start when you are 5. So if you’re not sure what “grade” he is, you could count up from there. That said, don’t get hung up on grade level too much. 
My 12 year old daughter is a 7th grader. Here is what she does daily:
Apologia General Science
Math
Latin
History (Ancient Civilizations and the Bible)
Literature selections
Our Mother Tongue
Spelling Wisdom 2
The Creative Writer
Wisdom for Life proverbs Bible study
SCM middle east Geography (they do this daily at our house, not weekly)
Fallacy Detective(1x per week)
My 11 year old son does all of those same things except different science program, different writing program, different Bible study. In addition to this, they both have other personal development books they read weekly and some church history books.
For my kids, I think I would be requiring a lot more for them at age 15 than you are requiring. HOWEVER, THAT DOES NOT MEAN THAT YOU SHOULD BE REQUIRING MORE. Every child, family, situation is different. I am only telling you what I would do.
I am not fond of Simply Grammar and I think that a 15 year old boy would probably hate it. But, like I said, every child is different. My son wanted to try Our Mother Tongue and it has worked out well for him, though it is a little challenging.
I hope this is helpful to you.