What ever treat you decide on; definitely not Karo on your finger, as that's just asking to start a bad habit of biting. A little bit of his normal ration in a shallow bucket would work just fine; but as you are lovin on him, the loving should be reward enough.
I don't really believe in treats for horses of any age, unless they are terrified of you. If you have a friendly horse, some good scratches should be plenty reward for being good! Animals do not need "treats" it only makes the owner feel better.
Giving a foal treats will just train YOU to give treats as it gets older, and then you will find your horse thinks you are a treat-dispenser and will bite at you, and only want your attention for FOOD. They will not want to see YOU they will just want the FOOD. I think that is a horribly frame of mind for a horse. I've seen horses that get lots of treats, and to me they seem very ansty, kick the fence, and bob their heads to try to get you to give them a treat. It just bothers me. My horses nicker to see me and are eager to get out and about.... but all I do is groom them and exercise them. I'm NOT the treat lady.
I also will echo the sentiments about not hand-feeding treats. Only give treats in a bucket or feed pan. That way they will not equate hands with biting and food.
Also, with showing halter, I have found that baiting with food actually makes it worse... the horse will not hold it's head up properly and creeps towards you because if it's not close enough to you then it feels that it can't get the treat, which is it's sole motivation. All it needs to do is be TAUGHT that it must set up and show, that is the job at hand.
Anyway, off the soapbox...
If you have a foal that is terrified of you and you need to motivate it to come towards you or you want to bait it to do something, just a handful of it's regular grain should do. Most foals don't have a taste for "treats" until they are older.
Best that he stays outside in the barn.
Yes, walks are great way to bond and safe form of exercise.
Continuing with the feed program the seller has him on his a safe way to go; but in general a good quality grass hay and a mare/foal or growth feed is a good choice (some do very well on a ration balancer instead of the mare/foal feed).