You need to get him halter broke, like, yesterday. It is possible to singlehandedly medicate a foal but to start off you do need to hang onto something. I had a colt last year that needed oral antibiotics for three months…add to that ulcer meds and probiotics and he was getting oral meds 3x day.
Getting the halter with a grap strap on will be first priority. Either do it while he is down and sleeping (be careful to get out of the way if he scrambles) or get help.
First time haltering with a helper…try to position the foal so that he hindquarters are perpendicular to the mare. This is a two person job. One person will have one arm aound his chest (not his neck, too vulnerable to damage) and the other around his hindquarters.
Once you have a breakable halter with a grab stap on I would leave it on until he gets more accustomed to the handling.
Life will be much easier if you make the meds very tasty. Here is what I did:
Find an old large sized syringe, like for probiotics or electrolytes.
Put required number of pills in a snack sized plastic disposable baggie (like a sandwich type bag but much smaller)
Add about quarter cup of sweetened applesauce (or enough to dissolve the antibiotics). Once dissolved (takes a few hours to fully dissolve) take your syringe and make a medication “parfait”.
First layer, in the nozzle, is a layer of corn syrup. Then cut little hole in the coner of your plastic baggie and squirt in a layer of medicated applesauce. Alternate layers and end with corn syrup.
Colt was not entirely keen the first couple times but pretty soon he slurped it up quite happily. After about a week I didn’t even have to put his halter on, when he saw me with the syringe he would open his mouth and just go for it. Take your time, don’t try to squirt everything at once. Small squeeze, let him swallow, another small squeeze, etc. Don’t rush.
Also, since interaction with humans has probably been less than wonderful, try to take time to just hang out with him when you can, find all of his itchy spots and spend time scratching him.
I am a one woman operation so I prefer to find a way to make it easier for horse and human and I was able to medicate him by myself for three months.