As for drinks, I would just give him water and maybe some herbal tees. I used rice milk for cooking/baking, but the stuff is sooo full of sugar and highly processed that I would NOT use it as a milk replacement, nutritionally. Are you still nursing? If so, or if you think he will start up again once new babe is here, that is all he really needs (you would need to eliminate dairy and soy, too obviously).
I would also be really cautious about including the soy lecitin or oil, at least at first. Get him totally cleared out first and see how he improves. Once you see what healthy is like for him, then you can test the soy lecitin or oil. And the same thing goes for trying goat's milk or raw milk with the dairy. I've found that observation is really the only reliable way to figure out allergies/sensitivities for young ones. None of my kids have had allergies show up on blood tests, but ds2 certainly reacted immediately to the most minute traces of dairy or soy in things (although he could tolerate things made on the same equipment as soy).
We also had a lot of success with constitutional homeopathy. Ds2 no longer has food sensitivities. It is a slower road, but does lead to true healing...and taralv, my ds1 was having chest tightening attacks that the correct remedy/dosage completely resolved... as has his eczema (though that has been a much slower fix). I never figured out what ds1 was reacting to, but the homeopathy helped him anyways (although it's obviously better to identify and eliminate the source). We also cleared up ds3's chronic cough as a baby without meds, just homeopathy. A professional, classically trained homeopath is essential, though.
Melissa , wife to Tom , mom to The Baron , the Bean , Little Bear , and Baby Beaver