My son, like yours, would not eat peanut butter at all when he was younger. I didnt want to push it and cause his aversion for peanut butter to become something bigger like an intolerance or allergy. So when he went to kindergarten and their lunch shift was the last (they had a snack break mid-morning) I didn't feel comfortable sending cold meats, etc.
So what I did was give him nutella sandwiches. As the year went on, I gradually mixed more and more creamy PB into it. So by the end of K he was eating nutella flavored/colored PB. 1st grade he still preferred nutella over PB but I bought it less and less (and it had more PB than Nutella in it then). Now he actually prefers PBJ sandwiches over cold cuts or other lunch foods (3rd grade now)
So in that sense, Nutella was great for us. It was his "training wheels" so to speak for PB.
Also, sugar BY ITSELF is NOT healthy, but sugar with protein which is longer lasting and filling is not as bad. (I've forgotten and don't have any nutella on hand now to look at, so I don't know how much protein Nutella has, but probably some, although not as much as PB??)
If there's no protein, all "white" carbs, then their blood sugar spikes and crashes. 'Good carbs' (ie wheat pasta as opposed to white, sweet potatoes as opposed to white potatoes, etc) lasts longer in the body and tides you over the sugar spikes.
I wouldn't live on nutella, but I'd still have it.