Hey, that’s good news that your stallion is looking better today after getting wedged between the rocks!
You’re not dealing with any kind of contagious pathogen or parasite in this case, so feel free to use your own judgment and return the seahorse to the main tank if you feel his scrapes have healed sufficiently, sir. Even if he should develop an infection at the site of one of the scrapes, it will be an opportunistic invader attacking the site of a mechanical injury, and not something that could be a threat to the health of your other seahorses.
As a first-hand observer, you are in the best position to determine whether the affected seahorse is looking healthy enough to go back with the rest of the herd. If that’s what you want to do, you could always use the antibiotic to gut load some live adult brine shrimp, and feed them to the seahorse in your main tank as an added precaution. That way you could get some antibiotics into him and help him stave off any possible infections, and still return him to the familiar surroundings with the support of his tankmates where he would be most comfortable.
That was good work rescuing your stallion from his predicament yesterday, Don! Here’s hoping he makes a complete recovery and is none the worse for wear after his ordeal.