My daughter was an absolute horror when it came to taking her medicine.
She did a little better with the bubble gum flavored stuff, but what actually worked best was letting her use the syringe herself so it wasn't like I was shoving something in her mouth. It took a little longer because she would take a little bit then have a sip of 7-up and take some more. But, it all got down her that way.
She actually started taking medicine in pill form at a very early age because she didn't mind pills, but anything liquid, ugh.
I tried hiding it in juice, offering her a treat after....
One day, I just offered to let her do it herself and it worked a lot better.
Maybe try that with your son.
They also have little spoons for medicine that have hollow handles and measure the amount of medicine on the side.
Your son might take to one of those better. If he had a surgical procedure, anything that remotely resembles a syringe might be upsetting for him. You can find those little spoons at any drug store.
I wish you the best because I know what you're going through.
My son would take any medicine with no fussing, it would just be over and done, but my daughter was a whole different story. She ended up with more of it on her than in her and she cried and fought.
Taste the medicine. If it's really bad, see if you can get something that tastes better for kids.
Try one of those spoons and/or letting him do it himself.
Things might go more smoothely.