I think ISFJ.
I:
- He seems to like being one-on-one with a family member or close friend, rather than with large groups, or unfamiliar people. That is characteristics of introverts who love people. They love them one-on-one, and prefer depth rather than breadth in their relationships.
S:
- read non-fiction only
- not interested in theories without application
- being artistic can fall either way (S or N), depends on whether he'd rather draw from imagination, or from reality
F:
- his sensitivity
- values close relationships
J:
- diligent, ambitious, and hardworking
- I know you mentioned he's flexible and laid-back at home, but P's have trouble being the way he is at work. Perhaps he just doesn't feel the need to be on his toes quite so much at home.
I agree that the J/P is the tough one though. Here are some tiebreakers for you:
- Does he like decisions to be made (J) or options to stay open (P)?
- Does he like being well ahead of the deadline (J), or is simply making the deadline enough (P)?
- If no one set deadlines for him at work, would he still set his own (J) or just let it get done when he gets to it (P)?
- When studying, how ambitious and diligent is he with subjects he enjoys less?
- If he uses planners, to-do lists, etc. at work, does he enjoy doing so (J) or does he feel like the amount of tasks has forced him to do so (P)?
- If he starts a task does he like to finish it (J) or does he tend to wander from project to project (P)? This can also apply to starting a painting, learning a musical piece, etc.
- Product-oriented (J) or process-oriented (P)? For example, when it comes making a work of art, does he enjoy the finished product a lot (can't wait to see it complete, may hang it up, admire, etc.) or does he enjoy the process more (and may shove it in the closet afterwards).
It would also help us to know what type you are. We tend to notice the ways others are different from us faster than we notice the ways we are alike. So you're more likely to err on the side of typing him as the one that you are not, on each letter, if that makes sense.