Hi there!
Keep doing water changes, as you know, goldfish are considered dirty fish, producing more ammonia than any others. So, if you have the time to do daily 20% water changes, or even 25% every other day, it will really help the fish heal. You can use Melafix and Pimafix, which are mild medicines, but they only really help if the water is kept pristine. Another thing to think about is that goldfish tend to get tumors a lot more than any other fish I have seen, so if it is growing, or looks like cauliflower or anything like that, it could be a tumor. I can't tell from the angle of the picture, sorry. Also, since you said the hole is healing, check your gravel for anything that is shapr that could have injured him. For example, when I set up my 125 and put the gravel in, I noticed broken glass pieces, and has to meticulously go through the gravel the get them out. The previous owner broke his glass pipe, (for wacky tabaccy--I KNOW!!!!), and put the pieces in the water because he thought they looked cool(OMG WHAT AN IDIOT, RIGHT?).
I know, I was so angry!! Anyway, look for sharp rocks, sometimes the bases on the plastic plants can break off and get sharp too. I hope he continues to recover quickly, and good luck!
Maribeth