I'm going to add a little contrast to the other answers. I think you should insist, but carefully. Focus on having your son set goals, encouraging your son / being there for him, setting a routine, and possibly finding a better teacher.
I have been playing the piano for more than 10 years, and I never would have made it this far if my parents did not "force" me to practice. At this point in my life, I absolutely love playing the piano; I happily practice for hours on my own. But it was not always so.
When I first started playing the piano, I was very determined to learn. While my mom was the one telling me to practice, I was happy to do so. Why? Because I had a goal: surpassing my sister. Which later became surpassing my brother. This is very key. Goals are extremely motivating. Now, even with that goal, I needed prodding and help from my parents. My mom was still the one telling me to practice.
There was a time where we kept a chart of practicing. If I practiced 15 minutes on a given day, I got to check it off. Enough checks resulted in a fun little reward. Give him something to work towards.
Encouragement / Always Being There
My mom didn't just send me off to practice. When I was practicing, she was always right there next to me, not actively helping me learn (after the first couple years, she could not because she is not musically talented), but she always sat there in the same room as me. Without fail, if she left to go do something, I would stop practicing. And when she was there, she could hear my progress and pointed it out to me, giving me encouragement.
A routine was very important to getting me practicing. I was homeschooled, so during much of my childhood, 8-9 AM was the practicing time (naturally, the length of time depended on how old I was). I knew that when 8 o'clock hit, I needed to be practicing the piano. I'd often practice without prodding from my parents because of this routine. Routines work wonders.
Fast forward a couple years, and I was taking a class that started at 9 AM, requiring me to leave the house at 8. This threw my routine out-of-whack. During this time, I rarely practiced the piano. I knew I "had" to practice the piano every day, but struggled doing so. Until I found a new routine, practicing was very sporadic. I might practice as soon as I get home at 2 PM, or I might practice at 8 PM, or at 5 PM - there was no schedule behind it. Eventually, I set a new routine that I would practice at 5 PM every day, the same time when my mom started cooking (our kitchen is next to the room with the piano, so she could still give encouragement and I still felt her presence). When I set that routine, suddenly my practicing became regular. It became rare for me to miss a day. Note that a set time works much better than something like "right after you come home from school" (assuming that that time actually changes).
At some point, who your child's teacher is actually matters. It becomes hard to improve once you reach a certain point (actually, there are many points where this happens). Practicing becomes frustrating because it feels as if no progress happens. At this point, a good piano teacher can be really encouraging. I know a certain piano teacher who, when I took lessons from her, managed to get me extremely enthused. I would practice without care of how long I was spending. And no matter how little progress I made, I felt appreciated by her; she'd always be encouraging, never saying negative things, instead telling me "how to make it even better." This same teacher has done the same with kids that I was teaching (way more recently); I couldn't figure out how to get them excited for the piano, but as soon as they switched over to her, they loved playing.
When I was struggling with continuing the piano, my parents did force me to continue a bit further before deciding. I am so grateful that they did this. If they did not push me past the rough spot, I would have quit playing the piano, yet the piano is such a big part of my life. I absolutely recommend that you insist that your son play the piano, but do so smartly. Do not randomly send him to the piano. Set a routine with him (he should know when it is practice time). Be there with him (it's his job to practice, but you are still there supporting him in it). Have him set goals (perhaps he should find competition with other young pianists; this might be hard until he's older). And just be overall encouraging. Try to make practicing the piano fun.