My son suffered LI for the last 10 years and every year we had problems with food / milk both with the airlines and resorts. LI is one of the lesser known dietary requirements, even most airlines don't cater for them.
People like vegetarians are far more catered for (which is a laugh really because being a vegetarian is a choice, being LI you have no choice - it's medical). The same applies to celiacs - ask the airline this one next time you book !
In the end we just to carry lactase liquid or tablets around and let him eat "normally" - Spanish soya milk I would not give to my worst enemy !