Disclaimer: I Do Not Own Agents of Shield.
He can't find a word strong enough to express how he feels about her because there isn't one.
What he feels is stronger than friendship, purer than lust and less selfish than love.
No, there isn't a word but he doesn't need one and she doesn't need to hear one.
She understands what he feels because she feels it too.
So things between them are not labeled or spoken about because neither one needs to be.
All they have ever really needed was each other and they have that even when they have nothing else.