I'm sorry, JS, but you likely won't like my advice. I'll give it anyway and hope that you will at least consider it.
Your son is too young to take piano. That is one reason he is not interested in it. His little fingers are too small, not strong enough, and he has the focus of a 5 year old - NOT the focus of a child old enough to play the piano. The better age to start piano is at 7 years old.
Just because the teachers are Julliard graduates doesn't mean that your son should be taking piano this young. From your description, he shows NO signs of being a great pianist in the making. It really sounds like you want to turn him into a concert pianist.
Another issue - Suzuki violin and cello are good programs. However, Suzuki piano is NOT. I am a musician and pianist. Suzuki piano teaches rote playing and most of the students can't really read music. Their hand placement on the keyboard usually looks terrible. It's about as bad as the old John Thompson. Yes, that's my opinion. I've worked with many teaching methods. Give a Suzuki piano kid a new piece of music that's relatively easy, and they can't sight read it. That's not a good way to teach piano.
Unless your son shows amazing talent, you need to ratchet YOUR dream down and let him be a kid. If he wants to start violin and you are willing to rent one, fine. But put your dreams of Carnegie Hall aside. If a child doesn't actually want to spend an hour a day practicing, (and btw, my own son with perfect pitch was one of them and I accepted that) they won't become a great instrumentalist. (However, my son sings and that's good enough for me!)
To keep him interested, switch to violin. Re-introduce the piano when he's around 8. If he still doesn't like it, forget about it. Torturing him now as a 5 year old isn't going to make him like it any more when he's 10.
Dawn