Depending on more facts, you might have a case. Report the incident to the hotel staff, take photographs of the shower showing the absence of rails, a mat, and anti-slip material, and most importantly consult with a personal injury attorney right away so that you can maximize the opportunity for monetary compensation.
I am sorry this happened to your son. You should consult with a personal injury attorney RIGHT AWAY to dicuss what happened and to determine the best course of action for your son. McDonald's will have its representatives. You should have a representative as well: use Avvo and find an attorney!
While you under the doctor's care, it is the best practice for the attorney to reach out to his/her client every month or so to see how they are doing and to see how the treatment is coming along. By doing that, the attorney can get to hear how the treatment is or is not helping and can thus decide whether to send you to another doctor or perhaps to seek a referral for more aggressive care. This should ultimately lead to a better "work-up" of your injury.
You will be suing the other driver, NOT the insurance company. As a courtesy, you may want to let the other driver know you will be suing him and the reason why (the insurance company is lowballing you).
That said, it is STRONGLY advisable that you hire a personal injury attorney to represent you. The other driver has his/her representatives to advance their interests; why shouldn't YOU??
If your insurance company reasonably had an opportunity to settle with the policy limits and failed to do so, and a jury awards the other party a verdict in excess of your policy limits, your insurance company (and not you) will be responsible to pay the entire verdict, including the excess amount.
To echo the responses of others, it is a mistake to handle this yourself. The other driver has representatives to advance and protect his/her interests; you need a representative to advance and protect YOUR interests. Hire a qualified attorney an be sure to get the compensation that the law entitles you to.
Consult with a qualified personal injury attorney right away so that you can discuss what happened. The attorney should be able to tell you whether you have a case. The grocery store will have ITS representatives to advance and protect ITS interests. You need an attorney to protect YOUR interests!
California law says that the person that caused the accident takes you has s/he finds you. That means that if you had an underlying and dormant condition that was aggravated, then that wrongdoer is responsible for all the harm that results to YOU. It is recommended that you hire a personal injury attorney IMMEDIATELY so that you can find out what legal rights you have.