Hes not "probably" a campbell's russian dwarf, he IS one.
Rodents like to lick salt off of hands, or sweaty parts of your body.
I wanted to add, that bedding in his cage is pine shavings, and pine shavings are bad for small animals. The only safe wood shavings are aspen, and thats because there are no dangerous phenols and oils that have been known to give rodents upper respiratory infections.
As for salt licks and mineral licks, avoid those. Those are not recommended to put in their cages!!
And about spinach, give in moderation. It can give them diarrhea, especially if they are not used to veggies.