If you do decide to quarantine and medicate the puffer, keep in mind that he will likely get it again when you put him back into the tank. Now that ich is in your tank, the only way to eradicate it completely would be to quarantine and medicate ALL your fish. Then you have to leave the tank empty for several weeks to ensure that the ich parasite has no host. That is just not practical for most people.
Ich is an opportunistic parasite that is present in almost everyone's tank (unless someone has quarantined and medicated every fish they've ever had before adding it to their display tank). Stress brings ich out moreso than anything else. Most fish will recover from ich on their own, if you do your part and minimize stressors as much as possible. This means ensuring high water quality, eliminating fighing/territory battles between fish, providing an environment that makes the fish feel secure and safe, and feeding a healthy diet.
Garlic extract is an appetite stimulant and boosts the immune system, but garlic will not CURE anything. Garlic extract will help your fish fight off ich, but it won't get rid of the ich.
As long as your fish continues to eat, I suggest you leave it alone and try to let it fight the ich off on its own. Some fish very easily fight it off by themselves (my trigger gets it once in a while), whereas some types of fish are extremely susceptible to ich and usually don't stand a chance once they come down with it (like a lot of tangs).