First, I'd get rid of the "he's so bad" mentality; horses aren't "bad." Horses act based on previous learned experience based on what has worked or some other medical/physical problem.
Eleven-weeks-old (two months) is still a baby and many babies tend to be a little mouthy and/or bite-y. I'm not saying I condone it, but rather it is "common" until taught otherwise. It's similar how many human babies put everything in their mouth; it's how both human and horse babies explore their new world. With his age being a factor, babies tend to have shorter attention spans. When you are messing with him, try keeping the sessions short and sweet. Don't expect too much too soon; let a horse be a horse.
How are you teaching him to lead? Does he know how to lead? How are you leading him? If you are simply trying to pull him along, he probably doesn't like that or possibly doesn't understand and will bite the lead rope in protest. I know a lot of horses (even adults) who try to bite the lead rope if they are being pulled along and/or are confused.
Is he actually biting people with his teeth or just licking and nibbling a little? I know many people would correct both, but they are two different things and should be handled differently. I'd highly discourage the biting of people, but try to be a little more sensitive when he plays with the rope or other non-sentient things. I know not all, but some horses do naturally grow out of the overly-mouthy stage.
Is he turned out? If not, can you turn him out? If so, is he turned out with other horses or a companion? If he is constantly in a stall or turned out completely alone, he may be bored. You trying to teach him is probably the only fun and/or different stimulus for him, which is why he could think it's a game. (Actually, even some adult horses play with the lead rope when they bored, not just foals.) Anyway, if he is turned out with an older, more experience horse, they could help teach him a little respect.
Another thing you have to do it reward the good. When he is behaving himself, do something nice to/for him, such as scratching his butt (if he likes that sort of thing) or simply leaving him alone (if he isn't a people-horse).
If he is bored, you could try getting durable, bite-able horse toys, such as jolly-balls. I know some foals (and adults) get an absolute kick out of those.
I do not mean to over-generalize, but some people (not saying you are one of them) tend to be over-sensitive with their orphaned foal. That "oh, the poor little baby has no parents and I had to save it" mentality - one they hold on to for the longest time. What may seem like a "good smack" to you, he sees it as playing. I understand young horses are more playful than other older horses, but if he thinks you are just playing around after you smack him, then your correction wasn't good enough - wasn't clear enough. I'm not saying to beat his butt after he bites, but you need to make it without a doubt, to-no-uncertain-terms clear that biting humans is not unacceptable and not the right answer. You also have to be consistent.
Here is a video of a (mother) mare correcting her foal around the 0:23 mark. Her little colt has absolutely no confusion about whether or not if she is playing around; he got the message loud and clear the first time.