Parenting after divorce; child prefers dad’s

My partner and I split approximately 15 months ago. We share custody of our five year old son and 2.5 year old daughter 50/50. We live a block apart and are very amicable; we spend time together regularly, no hostility between the two, we are very much on the same page. However, my son seems to much prefer staying at his father’s place. Yesterday he spent the night (first of our week together) and this morning he asked why he couldn’t go back to his dad’s tonight. He always says he would rather do things with his daddy. I am grateful for their good relationship but part of me is definitely hurt (I know that’s unreasonable). I have tried carving out one on one time, but after we do something, he just asks if next time he can do it with daddy. He asks how many sleeps until daddy’s. He talks about his dad constantly. Is this just a phase? How do I work through it on my end?