It kind of sounds like YOU think it is unattractive. I would be very careful not to let that thought out -- I think it could be pretty hard on a kid to think their mom thinks they need to do something about their eyebrows.
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That said, if he initiates it and wants to do something about it, I think it is fine for you to help him. I'd just say, "sure" and not make a big deal about it either way.