My younger DS used to projectile vomit ANY medicine that I tried to give him, including pred which he HAD to have for his asthma.

So on the advice of his doctors, and the chemist, from about the age of 18 months (when he was the size of a three year old) he has had tablets, usually half the adult dose (in accordance with the recommendations of the dr/chemist), in a teaspoon of AktaVite/Milo. Raw. It's all chocolatey which hides the taste, and he won't spit it out.

Check with your dr/chemist if there is some tablet or other method. Otherwise there's always suppositories...