oh i feel you on this... my 3 year old was like this... our trick was to sit her in the corner of teh couch get the syringe to the side of her mouth and near the back of the tounge and squeeze on her cheeks to get it inso she couldnt close her mouth.

now she take medacine with a great deal of "you have to" talk and big song and dance after she takes it so she knows shes been a good girl.

Mix the syrup or medicine in 50 ml glass of water and give it to him. Simple as it is. next time ask the doctor if there is flavored version of the same composition in the market. Children like fruit or mint flavored medicines.