This chapter is amazing and I can't wait for next.
From what we know about stand powers a stand can develop new powers over time, this is especially true when we are talking about kids, if you remember part 3 when Polnareff was turned into a small child by Alessi's Seth Silver Chariot was different as well. Also Echoes evolved and changed its powers during part 4 or Tusk in SBR and many others. The kid is also under a lot of pressure, with the looming threat of the disease, his father forcing him to lie, his grandmother reappearing after years and now the bullying. It's a lot for a kid this young and the stand could be going out of control because of it.