It is good that you caught him fairly quickly, some flighty ewes of mine are very difficult to catch, even with flystrike... So then have to get the WHOLE flock in just to treat one...
If you get a small bottle of crovect then you could also treat this yourself, must be quite expensive to get the vet out. I use a spray from the vets, just spray it over the area as you scrape the maggots off, they die very quickly. then dagg the area, and another spray. Works very well (use disposable gloves).