The 17-year-old joins Hinshelwood’s squad to replace Roco Rees, who picked up a knock on his left foot while training with Brighton.
Bull will be available immediately for use in today’s match away at Kingstonian.
The young Englishman is highly thought of at his parents club, and will fill in while Rees is out injured. Roco’s knock is minor, and he is expected to be back in action imminently.