Your horse needs to reste for at least 2weeks (the time for the scar to heal) but you can extend this period to a month to be sure.
You can put him in a stable or in the field, it depends of the character of your horse and also the period (summer: lots of flies, better in the stable), the more he'll walk, the better it'll be for the healing. In both cases, you have to be very cautious with the hygiene of the place (make the stable every day, be aware of the branch in the field...).
For the care, watch the good evolution of the scar everyday (every 4 hours during a week is the best) and use disinfectant if needed. You can give him anti-inflammatory during a week but you should see it with your vet.
Then, you should walk him in lunge everyday, twice a day during 20 minutes each time, you can shower him once a day too and if you have a paddock (without mud or dirt) you can put him a few hours per day in order to let him walk.
Finally, check his temperature everyday during a week in order to notice if there is oedema.