i currently have two fancytail goldfish....i cant afford a bigger tank for both of them so i have to give one away....do fish get lonely or do they not know the difference...
In a small tank, one goldfish will be much better off than 2. Unless you have at least a 29, I'd get rid of them both--one fancy is not suitable for a 5 or 10 gallon tank.
You could get 8-10 white clouds, no heater required, and they have interesting behavior. Or maybe a half dozen zebra danios for hyperactivity...simple, active, hardy...good beginner fish for a smaller tank...enjoy....
If you can only get your goldfish a 10 gallon, you really shouldn't add any more fish.
One reason goldfish need such a large tank is because of the amount of waste they produce. You will be over-taxing the biofilter by adding more fish to the mix.
Health-wise, he's probably better off as the only fish in the tank.