Should I ask my co-author before I upload an article (that he already told me is okay to publish) to arXiv?
Yes, you should check with any co-authors before publicly posting a preprint of work that is not yet published.

They may prefer that you not post it at this time (e.g. if they have a particular timeline in mind for when they want to publicly share the work, given other related things they are working on). Even if they have no reason to object, you should check with them as a courtesy.