Romeo Smith's mother said her nine-year-old son had been bullied all week by a group of older boys before the attack at about 21:15 BST on Sunday.
Natasha Smith said Romeo was assaulted in Ilion Street, Mansfield, and the plank needed removing in hospital.
Nottinghamshire Police said the assault is being reviewed by its youth offending team.
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Romeo's father, Craig Smith, said he saw his son being confronted by two boys.
"As Romeo was walking away one of the lads came running up behind him and threw a bit of wood with a nail in it," he added.
"I saw it just stuck there, in the back of his head, and then Romeo started screaming and running towards me."
Romeo is autistic and his mother thinks this may increase the long term impact of what has happened.
"Now I feel I can't let him out on his own because due to his ASD [autism spectrum disorder] he is very vulnerable.
"And this has been happening and he hasn't even told me about it - I just feel I wouldn't be able to let him out because it could happen again and he probably wouldn't mention it," she said.
Nottinghamshire Police said the incident could be dealt with "via the restorative justice route".
"We recognise that children sometimes do things without considering the consequences or the seriousness of their actions," the force said in a statement.
"In cases such as this, where genuine remorse is shown and there is an understanding of the consequences of their actions, we try to mediate between both parties to avoid progressing down the criminal justice route."
A boy was left with a nail embedded in the back of his head when an older "bully" threw a plank of wood at him.