Your horse may very well be licking you for the salt that is on your skin. Most people will have some level of salt on their skin and even more when they are sweaty. Salt is comprised of two minerals, sodium and potassium, both of which play important roles in maintaining overall good health. Your horse may just like the taste of the sweat/salt that is on your skin. Some horses will seek our salt or minerals, by eating or licking unusual objects, when they have an electrolyte imbalance.
Electrolyte imbalances can occur due to hot weather, dehydration, excessive sweating, diarrhoea, heavy exercise, illness or poor nutrition. Signs of an electrolyte imbalance or deficiency can include; poor performance, abnormal sweating (excessive or reduced), poor coat condition, muscle problems like stiffness or tying up or slow recovery after exercise.
If your horse is licking you and appears to like the taste of salt you could offer a salt or mineral lick block that he can frequent as he pleases. Something like the Minrosa Salt Blockmight be appropriate. There are also a wide range of electrolytes available for horses to support their mineral needs. If your horse is showing signs of an electrolyte imbalance or you have concerns about their health you should discuss this with your veterinarian.
Of course there is also the possibility that your horse licks you just because he likes the contact with you or the feeling of it. There is also the chance your horse is just taste testing so beware the next time he might take a bite