Heather, first of all, I'm not a doctor or physical therapist, so I can't give you any type of medical advice!
I can say though that, based on my experience as a parent and based on my many years of babywearing, I highly doubt that you've caused this issue either by the way you pick up or hold your son in your arms or by babywearing. It doesn't really make sense -- I'm right-handed, and have always worn my babies on the left in certain carriers like slings -- but they've never had an issue with only looking to the left. It seems to me that there must be some underlying physical cause entirely unrelated either to your handedness or to your babywearing, because otherwise we'd see a lot more babies of left-handed moms looking only to the right and vice-versa!
So please don't feel guilty (or let the therapist make you feel guilty) because it's just so unlikely that there's a causal relationship between babywearing and the way your son turns his head. Definitely it's good to get tips from the therapist in terms of ways you can gently help correct the issue, but I don't think you need to worry about whether continuing to babywear will be harmful. You can certainly practice tying the wrap in different ways to help encourage him to learn to turn his head the other way -- that's the beauty of a wrap, there are infinite tying possibilities!
But I think it would be really worthwhile trying to get to the bottom of this with your health care professional. It could be torticollis, or it could be something else as the posters above mentioned -- but my guess is that there is something physical going on, whether a muscle tension issue, or a vision issue, or whatever it might be.....I definitely would talk to your pediatrician and seek a second or third opinion if you're not getting answers!
Please keep us posted, and please keep wearing your baby!