Yes. If you bought the bike assembled by an employee of a sporting goods store, you may sue the store for its negligence in assembling the bike. Consumers often buy sporting goods already assembled, such as bikes, and do not suspect anything is wrong with them. Unfortunately, mistakes do happen, and sometimes, parts are not assembled properly. In your son's case, the training wheel was likely not properly assembled. Therefore, the sporting goods store may be liable for your son's injuries. In addition, your son may be entitled to the following:
Suggested Reading: Dangerous Sports Equipment Cases - What You Need To Know
If you would like to discuss your son's legal rights and his options, feel free to contact me @ 877.944.8396.