A rasp can be used as well. The important thing here would be to trim the tip and round it properly so the rooster will not gore himself.
For me using the dremel has turned out to be the best, quickest and easiest way with no bloodshed at all. There are various videos on youtube showing how to...
I want to applaud you for your effort and all the trouble you are taking on even though you are just housesitting and he is not your rooster! :thumbsup
@Xouie beat me to it. :plbb
These wounds might be caused by his own overgrown spurs as with every step he takes the sharp and pointed ends will cut into his own legs. that might be the reason these wounds cannot heal.
If this was my rooster, I would trim back his spurs using a dremel.
With spurs this long he will not be...