Ah yeah, I don't mean to say you can describe all modes of behaviour with sixteen 'types' very accurately. For example, my brother is an ISTP, I'm an INTP, similar on paper, but in practice... we're very different people.
Even the idea of personality is a little dodgy, I think - identity is definitely a tool, I think, that we use to help understand ourselves, but it's possible it may not be an accurate depiction of how we often function and may not describe how our behaviour can quickly change under certain circumstances. 'Personality' also presumes that we have a static identity and it usually doesn't leave us room to change.
But MBTI is just for fun, I think

It's an interesting experiment, and I like thinking about it.
I barely understand but I think INTP/J (sometimes called the Architect: theoretical, abstract, yet logical. And the Mastermind: sceptical, driven, playing the long game usually) might be right. There's just enough softness in him that you think he might be waylaid, but he's still logical enough to continue on anyway.
Or maybe INFJ (the Counsellor: insightful and organised, committed to their goals), who cares a bit more about people and feeeelings, but still has a clear idea of what needs to be done for The Greater Good.
Thoughts?
Those would be the three I'd label him with. I think he works more as an INTP than an INTJ, since INTJs tend to look for the application of knowledge, and see it as something they can test and use in the real world. INTPs are more interested in knowledge by itself, and will more happily dabble with abstract concepts and things which aren't immediately helpful, searching more for wisdom than solutions. 'Wisdom is its own reward' is very Solas.
That said, if we're interpreting Solas to be more feelsy than logical at core, then INFJ probably suits better.
I'm an INFJ (used to be INTJ as teen but when I hit 19 I discovered emotions), INFJ are usually described as very sensitive (-sobs-), emphatic, intuitive and said to have artistic tendencies. I think Solas has a front (Fen'Harel) that fits a ENTJ and behind that mask is INF/TP.
At the end of things this system is flawed because it cannot capture the complexity of the human mind, but it is far better than a lot of methods out there.
I had the opposite journey - I was a sweet lil' hippie child INFP who believed in the power of love with all her heart, but at some point I became cynical

Or rather, I realised that I was more interested in technical concepts than feels, but I often used technical concepts as a framework to understand feels, which made it very difficult to figure out what I was doing at first.