I had the same problem with my DS, he wont take ANY medicine, regardless the taste. So I googled and theres actually a lot of tricks.
One I found most useful is to squeeze his cheecks together and insert the syringe into the back cheek of his mouth and just give him a little bit at a time. Squeeze it until he swallowed it, I would rub his cheeks to help him swallow. It might take you few minutes to give him 5mls but it works! They wont like you doing it and will probably scream, you need someone to hold his in a craddle position.
Other tips I found was blowing into their face as they are swallowing (didnt work for me )
Tickle their chin (didnt work for me either)