This is a link to the TSA page about travel with disabilities:
http://www.tsa.gov/travelers/airtravel/specialneeds/index.shtm
TSA = Transportation Security Administration
As the others posted, it would be considered a "medical liquid" and is not handled the same as other liquids. Scroll toward the bottom of the link to see - basically, it says you need to separate out those liquids and declare them as medical liquids. Basically, it just means you separate them out by themselves and tell the agent you have them.
As was also mentioned, amount allowed thru the security checkpoint is a bit vague. Technically, all they have to allow is enough for the flight plus a bit of extra in case of delays. So, you would or be able to take a week's worth in carry on bags.
Then, there is the question of whether the Bahamas will allow you to bring the rice milk in.
You will have to do some checking about that and possibly see if there is any way to get it in the Bahamas. Is he an age that he could do without the rice milk for the time you are down there or could he use a powdered alternative for that time?
I haven't heard of the powdered product another poster mentioned, but it would be much easier to pack in the checked bag than liquid. If you pack powder or liquid in your checked bag, be very careful about weight. Those items are very heavy!