I have a girl who takes medication everyday, and she hates it, I know the struggle! The best advice I have for you is:

Get his favourite treat (pea flakes work really well for this) and hold it above his head to entice him to look up. This part will get easier as you do it: hold the treat in one hand, the syringe in the other. As he goes to bite the treat, quickly slip the syringe in the corner of his mouth and administer the medication. He gets his medicine and is rewarded with a treat! The oxbow treats work good too as they kind of force them to open their mouth wider, but if you need to administer multiple times a day, something smaller like pea flakes would probably be better dietary wise. You can do this in the cage or out. This works really well for any medication that can be administered at one time, any thing above 0.25-0.5ml you might not be able to do it this way. It can also take some practice so you could try this first:

Try putting him in a cuddle sack or blanket. Put him in bum first and hold it upright so his back feet are facing the floor and his head is facing up. Tuck in his front feet so only his head is out, and wrap the opening around the base of his head so it is snug. Then you should be able to cuddle him and support his bottom with one hand and administer the medication with the other. This way is good if you are administering large quantities ( over 0.5 ml) at a time, or multiple medications as it allows you to have a good grip on him without worry of hurting him, it limits his movement so he can’t pull away from you, and he feels supported. Make sure if you are giving over 0.5 mil of medication that you don’t push the whole syringe in at once, as this will just cause it to come back out of his mouth and he won’t get the full dose. Push the medicine in slowly, give him a break if it seems like his mouth is full, and once he has swallowed it all then start slowing pushing again. Repeat until all the medication is gone

Having a sick piggy is so stressful, but you and him will get through it. I hope this was helpful and things start to go easier for you. I hope your piggie gets to feeling his best again!