looks like it might be fungal, id say talk to someone at the pet store and look into getting anti-fungal treatment if they also agree. it would be good to isolate him too while being treated in case something happens and to prevent the spread to others.I had an adf die from a fungal infection and it was so hard to watch. I would also suggest a water change in the main tank and treat that water too to be safe. again double check with someone else but thats exactly what it looks like to me, good luck and i hope your little one does ok
