This is a problem many parents have experienced. Your child loves the idea of becoming accomplished in a certain area, but the work necessary to make that happen isn't so appealing!
We know from research that there are three things that contribute to internal motivation: competence, autonomy, and connection. Let's look at what you could do to foster each of these:
Competence: We all like to do things that we feel we're good at. Have you told your son how much you enjoy hearing him play? Is your son's piano teacher encouraging? Is he getting to play pieces that he likes and can master reasonably quickly? Talk to the piano teacher about your son's difficulties. Your son is definitely not the first beginning pianist to struggle! The teacher may have some good ideas about how to help your son feel better about his piano skills.
Autonomy: Turning piano practice into an ugly power struggle every night is a sure way to make your son hate it. Nobody likes to feel controlled. Is there a way you can give your son some choices about when or how he practices or at least a rationale that makes sense to him? Acknowledge that practicing can be frustrating, but explain that it's a necessary part of learning an instrument, and he will definitely improve if he sticks with it. Ask your child, "What would make it easier or more pleasant for you to practice?" Would it help to practice at a different time? Does he need siblings out of the way or a pillow to sit on? Would it be easier to play each piece three times rather than practicing for a certain length of time?
Connection: We like to do things that bring us closer to people we care about. Would group lessons be more appealing to your son than individual lessons? Would it help if you sat with him or stayed nearby while he practiced? Is there a "cool" piano-playing teen in your neighborhood who you could hire to supervise at least some practices?
It may also help to take a playful attitude toward piano practice. When my children were young, I used to make up silly words to their piano pieces and sing along or do ridiculous "interpretive dances" to their music, which got everyone laughing. Also, ask yourself if this is the right piano teacher for your son. One time, I took one of my children to a new music teacher who made a snobby pronouncement that he didn't believe children should be allowed to play music until they had mastered all of their scales. We didn't come back. He obviously didn't understand children. And, he didn't understand my goal as a parent, which was to encourage in my children a love of music.