Mine is a trotteur (racehorse but he wasn't fast enough so didn't take part in any race), probably separated from his mother too early, he licks many things too. Actually he used to, but as years go by he licks less and less things, but he still loves to gently lick my clothes, arms, even my cheeks, and he also used to take everything in his mouth... As he was young, I let him discover things this way, but now when he's supposed to work or focus on me he knows he can't and I just have to say "no" once and that's all. But he has another "habit", that is to suck his own tongue. I explained and showed it to 2 vets who said it had no health consequence, was probably due to a premature weaning, and he does that when he feels comfortable, after being fed, after a long massage, when he's been really afraid and I came to reassure him, after a treat... That won't disappear, contrary to the licking habit which decreases.
The licking habit decreases but I didn't really act to make it disappear, I only said "no" and had him back up when it was bothering me, otherwise I just ignored him. I think it all got better when he took his place in a group of horses, became their leader, and acknowledged me as a leader and someone next to whom it is comfortable to be (safe, strokes, games...).
Oh and he pretty often licks other horses too!