Looks like an ulcerating wound to me, not sure.   But what else can I see: all those tiny white spots all over his body and fins? Or am I seeing things?

TEST YOUR WATER ASAP, post the numerical results.
Make a waterchange accordingly. Add conditioner and beneficial bacteria.
Treat with Pimafix, Melafix according to bottle instructions ASAP.
You also may need some stronger medication, like antibiotics and medicated food to get rid of ICK ( this is what the white spots look like to me )

Minde has got a good treatment plan with epsom salts and rising the temp. She sure will be on soon and give you advice too.

just for a basic rule to remember:

ALL ILLNESSES COME MOST LIKELY FROM A BAD WATER QUALITY

Ugh, that is ich, with an ulcer.
You will either need to treat with a progressive salt treatment and heat or with medication.
I found a good fast way to treat ich.
If the ich gets into the goldfish's gills it will suffocate them.
You need to treat the whole tank since ich is a free swimming parasite.
FIRST  DO A 50% WATER CHANGE WHETHER ONE IS DUE OR NOT!
THEN RAISE THE TEMPERATURE OF THE WATER UP TO 86 DEGREES.
take out the carbon filter if any is being used.
DAY 1 Add 1 TBS of Aquarium SALT per every 5 gallons of water, dissolved in a cup of tank water and ADD 1 tsp of Rid ich+ per every 10 gallons of tank water.
DAY 2 Same as Day 1
DAY 3 Same as Day 1
DAY 4-5 ADD 1 tsp of rich Ich + per 10 gallons of water, BUT NO SALT!!!
check and see if there are any spots left after day 5 and keep adding 1 tsp of rid ich + per 10 gallons of water every day for 3 days after you see no spots!!!  This treatment could take up to 10 days.
I DO NOT CHANGE WATER AT THIS TIME BUT I DO CHANGE THE CARBONLESS FILTER EVERY OTHER DAY.
I USE FILTER FLOSS IN MY FILTER.  IT IS VERY DENSE AND WILL TRAP ANY FREE SWIMMING ICH PARSITES.
AT 84F THE ICH WILL NOT BE ABLE TO REPRODUCE.  AT 86 DEGREES ADULTS WILL BE KILLED.
AFTER 3 DAYS HAVE PASSED WITH NO SPOTS YOU MAY PERFORM 30% WATER CHANGES A DAY AND PUT THE CARBON BACK IN YOUR FILTER.

Hahaha that's funny Nossie because I totally had my suspicions that they stole it but actually had no idea 

Hope everything works out, I have to agree with Nossie about the water quality, it does seem a bit strange. Unless, have you added anything new recently?

Fish that are harboring disease may show no signs of it for many months but one stressful situation can just make everything come out.  Disease appears all of a sudden and you say "what the heck?"  Ich parasites can be breeding and swimming in your water and not affect healthy fish at all but if the fish become stressed watch out!!  That's why what andrea and nossie say about water quality is so important.  Test your water at least once a week, more often if your tank isn't established yet.
Bad water quality can lead to a fish's demise very quickly!!!