Iantooooo oh boy there's so many ways to look at their relationship. Jack cares for Ianto quite a lot. Above and beyond how he cares for the rest of his team. He does acknowledge Ianto as someone special to him - but based on the pattern of how things turn out for him (can't say that he was proven wrong, either) he tends to prefer the life of hopping off and flirting, not wanting either of them to take the relationship (if you can call it that) all to seriously.

Ianto's death is a serious source of guilt for Jack. Partially on Ianto's last words (to him - it meant Ianto saw it as more than just casual sex and dates), partially because he knows he was responsible for leading someone else he cared about right to death and someone who cared so much about him.

Jack's view on relationships is far more jaded than Ianto's and though he does love him, it likely isn't the same way Ianto loves him.