Your son could be charged with assault for grabbing someone in a headlock and handlocking their head. Often police refer charges to prosecutors without a full investigation as to what caused an assault and whether the person charged has a defense to the charge such as self defense. Assuming that your son does not have a record, jail is very unlikely. I suggest speaking with an experienced criminal defense attorney to discuss your sons options.
As the other attorney stated your son could be charged with an assault for merely reacting to this bully and grabbing his neck. I highly doubt this would result in jail or even a conviction for that matter.
Before allowing or refusing to allow the school to conduct a "risk assessment" on your son, it is important to discuss the ramifications of your son's statements with an attorney. Most of the time, a defendant in a criminal action can count on anything he says to another person being used against him. There are, of course, several exceptions to this rule, such as statements to his attorney. Whether or not similar protection applies to information given to the school as part of a risk assessment (unlikely) is something an experienced defense attorney can answer with more certainty. If the assessment is something that may be used against him, it may be a good idea to disallow it. On the other hand, transferring schools and disrupting your son's life may be avoidable depending on the circumstances of the assessment or its outcome.
The bottom line is that an attorney could help you with this decision, which should not be made lightly.