Dogs don't have the psychological power fetish thing that men do about getting their cocks sucked. And its not a natural behaviour for them, so if you're doing it, you're doing it for yourself. Generally, he won't seek it, although he will enjoy it.
I don't recommend sucking him to orgasm, or at least, don't do it too often. Especially early in the relationship, there's a danger of creating the wrong association or connection. Remember, he's learning to perceive and relate to you as a sexual being. Don't teach him that satisfaction comes from your mouth, or he may have trouble realizing that you have a cunt.
If you're sucking him, he may start to hump. That's usually a sign that he's ready to mount you. Humping is natural behaviour and dogs don't usually have the control to remain still for oral sex like men. So his inclination will be to hump or fuck your face. Once, my dog humped my face to orgasm, my throat was sore. Also, watch out for his come, there's a lot of it. So much that the first time he came in my mouth, I choked and snorted it out my nose.