Take it back to the pharmacy and have them add a flavoring to it - bubble gum is usually the favorite but they offer over 300. If that doesn't work, ask if it comes in sprinkles or medicine straw or some other form (like probiotics do).
My first was like this. Even if we got something down, within five minutes he'd puke it up. My two year old, thankfully, will take anything we give her but she also will take small pills just fine. Given that it's an antibiotic, you're probably out of luck, but check how large the pill equivalent is if you think your ds might be able to handle a small pill.
IF not, you may want to try scare tactics and reminding him that if he doesn't take it, the doctor will HAVE to give him a shot (confirm with dr this is a viable option).
Otherwise, you'll probably end up sitting on him and forcing it into a cheek with a baby squirter thing. Sorry!
Good luck!