You could try to sue the city, but bear in mind:1) The city would have to be at fault in some way--that is, it's not enough that the bolt was exposed, but it would have to be considered negligent, or unreasonably careless, to have that bolt exposed and the city would have had to have had some warning or notice of the issue.2) Suing a municipality is always more expensive and difficult than suing a private party--there are special procedural rules which have to be complied with.3) If  you son has (hopefully) not suffered some long-lasting impairment, disability, or disfigurement, all you could likely recover are your out-of-pocket (i.e. not covered by health insurance) medical costs.It is often not worthwhile suing a municipality, unless there are significant costs or losses involved.