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I would opt for a combination of Marvin's and Allan's suggestions. What about taking the shadow color of your son's head and continuing it into the background, but cooling it a little with ultramarine. Then you have shadow color consistent with the other shadows, and the background will recede.
A variation of this would be using ultramarine and warming it with the shadow color. This would also grey it, but not randomly. Both variations can be lightened with white.
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