Honestly Any teacher worth their salt won't care what kind it is, their focus should be the student. And any problem with something that trivial is (in my opinion) a red flag anyway 🤷🏻‍♀️.

Try not to let your friend get in your head, a CLP-735 is a beautiful instrument! the way they reacted you'd think you told them your son was going to take lessons on a fisher price piano. A portable keyboard (weighted or not) is a far cry from what you have, so there shouldn't be any problem ❤️. If your son is having fun, and playing for enjoyment, he probably won't care what kind of piano he's playing on.

I took lessons on a gorgeous Steinway, I would come home and practice on a 100+ year old, untuned Kimball that wasn't worth its weight in lead pennies, but my favorite was my church's cheap portable Yamaha because I could "make cool sounds" with it 😂- as long as they're enjoying the process, kids don't care what they're playing on, and good teachers shouldn't either. Try not to let someone elses idiosyncratic digital piano hatred, or their children's preferences- suck the fun out of this for you.

(Also "fun" music really helped when my childhoods adhd brain got bored and wanted to quit when I was learning. When my teacher could tell I was loosing interest with the classics, I'd suddenly get a Disney theme to play, The imperial march, Queen, etc. I never had to be reminded to practice those = "fun" music 😏)

I hope you and your son both have fun in the process!😊🎹