I wouldn't put him on cow's milk if he is having this reaction. You should understand that it is not an ALLERGY - but rather that he is missing something that allows him to digest milk proteins. You can add that through proper supplementation, or you can avoid the milk. If he's missing what allows him to process milk, adding milk (splash or otherwise) isn't going to fix it. And most people will tell you that the purpose of cow's milk is to turn a 100 pound calf into an 800 pound steer in 6 months - so there's no reason that people need to consume it. The problem will be with other dairy products that you want him to enjoy - yogurt, cheese, ice cream, etc. as well as not having to limit him in situations where he's going to come across dairy products (birthday parties, preschool, etc.). In that case, you'll want to work on eliminating the problem through supplementation.
I would either go to a proper supplement, or I would go to another "milk" product, either soy milk or lactose free cow's milk or rice milk, etc.
No matter what you do, it's going to cost money!! But it's better than diarrhea (extra diapers) and rash (extra creams) and doctor's visits (co-pays).