I have been in your situation. I told my farrier, “nothing against your apprentice, but I would prefer is you take the lead with my horse. He seems to do better with you”. Notice how vague that last part is. Gets you off the hook without offending anyone.
Remember there is a benefit to you from your farrier having apprentices. One day your farrier will retire (or–from past farrier experience–any number of other random and exciting things that lead to no farrier with no warning) and you’ll hopefully have one or two of these apprentices who you get along with to replace him. (You will also know which ones you don’t get along with–much better to learn that while they’re being supervised.)
I might say no more of this specific guy based on he gives you uncomfortable feelings, but agree with the above that the lameness doesn’t really sound like his fault.
I agree with all of the above and I would also not want someone around who I find at all creepy. I am not so sure it would be about working on my horse as it would be just having him right there, in my own “territory,” rather than in a medical setting. 
Forgive me if I am stepping over the line here, but I am dying to know what was creepy about this guy. :eek:
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