Homemade Soy Milk????

Good Wikipedia article -- http://en.wikipedia.org/wiki/Soya_milk.
Check on the availability of soybeans. If they are available, you'll probably have to buy commercial size bags -- and be sure they are food grade. From a little reading, I found a recipe to make 1 gallon, using 2 1/2 cups (probably a pound) soybeans. (If you want the recipe, send me a message from my cook's page.)
Substitutes can also be made from rice, almonds, oats... which might be easier to locate and make.
To me, one element of dealing with this would be to analyze how he uses milk. Drink it by the glass? Make custards or pour over cereal? If he can reduce his use of milk, the problem may seem less important.
Another possibility is to get lactase enzyme (either the brand Lactaid, or simple enzyme pills) or an enzyme blend for milk digestion (which will improve digestion of both the lactose and the protein in milk; Enzymedica is one brand). This is a solution that could be discussed with his doctor.

I think Food & Wine Magazine had a recipe for this recently. Just google "food and wine homemade soy milk". Good luck.

Yes, it's totally worth doing and not all that hard. Andrea Nguyen has just released a great book called Tofu, which also has the instructions to making tofu and it's precursor, soy milk. From what I remember when my sister did this decades ago, soak soybeans in water, grind it up in a blender with fresh water, cook this watery pulp (stirring constantly), and then strain out the solids (which can be used to make Korean ground soybean stew) through cheesecloth and squeezing out the milk (this for me is the hard part of making soy milk). Sweeten as you like. OMG, your own homemade soy milk is so sweet, even without the sugar.

Since I was spoiled by my sister many years ago with fresh soy milk, I can not drink that stuff in the store, unless it's one of those Vietnamese snack shops that make their own soy milk.