Sherlock can tell something is wrong but he can't relinquish his grip now. He keeps the boy pinned tightly against his chest with the one arm and finally finds what he's looking for-though it's a bit difficult with a struggling seven year old whose head is at serious risk of colliding with his chin if he's not careful.

"Hold. Still." He demands, gripping the chemical mixture quickly and uncapping the syringe with his mouth. He wastes no time in injecting just a little into the seven year old. He has to mind the dosage because to because too much is potentially lethal.