To me,
Quinton and
Quentin seem real and fine. I think
Quintin seems rhymey/singsong, and
Quinten looks feminized to me.
I like
Quentin the best. I think you could definitely still call him
Quin (but I think it'd be a little weird to spell it with 2 n's, since
Quinn is a different name).
Quentin Dominic,
Quentin Marius,
Quentin Albert,
Quentin David