Around 18-months-old, children develop separation anxiety. Even my kids, who love being with their dad, would throw fits when Mommy left the house or the room for a bit. Kids do grow out of it, though patience and acceptance is useful.
Also, if a kid demonstrates marked preference for one parent over the other, it's important to gently but firmly let the kid know that you (the parents) make the rules, not the child, and whoever is able to dress, feed, bathe, read to, or play with the child will do so. Over time, the child will learn to accept that his needs will be met, even if it's not the person he'd rather do it.
And, it can help if Dad is more involved in child care, rather than just play time. Our youngest had colic and my husband couldn't bear the screaming, so he and youngest didn't really warm up to each other until the baby grew old enough to eat Thai food - and then they bonded over food!