N.W.
Tell him the truth. You use the juicer plus it has sentimental value for you.
The son may have asked because he could put it to good use if the juicer is just sitting on the shelf. But it's not, you are using it so it doesn't even make sense for you to lend it back to him. One of you is probably going to have to buy a new juicer. And since you want back the one you have, it makes more sense for the son to buy a new juicer for his dad as well as his wife's new dietary changes.