Yes, human valuing always involves a human act of comparison between individuals. However, I'm wouln't analytically go so far as to say that the act of comparison implies utter relativism. And if I were you, I wouldn't assume that your son's handmade coaster would be utterly worthless to me. You don't know that. That's for me to tell you, not for you to merely surmise about me (and quite subjectively so I'd say, really).
Sure, I might not feel as strongly about it as you do since, well, your son is your son. But if I have certain common empathies, emotions and (eek! -- values) as you do already, I might say: "Hey, that's a dandy coaster your son made for you there! I know that's a special thing!" And why? Because I likewise have a son who has 'made' things for me, and I understand (by comparison even) that these things can be especially felt on the immediate family level.
In fact, I might even value your son's eyes more than you do; this might be the case if he were say, a world reknown brain surgeon and he was about to operate on me. I'd value very much his good eye-sight.
I think we can both agree that taking a moving survey of any worth among nearly 8 billion people is still impossible (today, anyway). So, when we talk about 'objectivity,' if it's real then it can't be expected to be something too dissimilar than what our individual, subjective experiences are and be somethat that, thereby, we are able to tap into, even if it's not on a universal level among ALL human beings in all places (or at all times).