Liquid, right? Hold the tortoise's head behind the jaws so he can't retract. Pry open the mouth and put the syringe way back on the back of the tongue then still holding him upright (head towards ceiling) let him swallow. If you let only a little of the medicine out of the syringe at a time, and give him a chance to swallow after each squirt, it works better.
Also, you can hollow out a strawberry or something like that and let the tortoise eat the medicine that way.
When capturing the head be careful, torts have less vertebra in their necks and can snap easy if not careful, same with opening the jaw, their lower jaw is not connected(the bones float), typically a little pressure along the mouth can get them to open.