It sounds like talipes (think that's how it's spelt) My eldest daughter had this when she was born, however hers was both feet and extremely noticable, to the point where she wouldn't have been able to walk without them being corrected.
She spent the first 4 years of her life in and out of plastercasts up to her thighs to correct it. It basically involved manipuating her feet to the correct position, then plastercasting them. It worked well to a degree, though even now at nearly 21 her toes still "drift" inwards.
I was told that her condition was down to how she was laid in the womb, instead of being mostly legs straight with knees bent, in a kneeling position she was cross-legged in there and this caused her feet to curve inwards.
If your son is walking well un-aided and it's hardly noticable, i'd be tempted to leave well alone (unless your GP recommends otherwise). As i know from experience that a toddler with a leg, or in my case, legs in plastercasts is a thoroughly miserable time for them.