S.H.
No I never experienced that with my kids. But, when they are just starting to walk.. well, they are not "expert" at it yet. So yes, they often times fall on their tush, or what not, which is normal.
BUT, I would for sure, talk to his eye Doctor and I would have him/her explain his vision problems IN FULL DETAIL to you/his Mom. I'm surprised they didn't tell you about what the ramifications are, of his eye problems. USUALLY this is standard procedure... explaining to the patient what and how their eye problems will affect them etc. AND IF THEY DO NOT WEAR THEIR GLASSES, I imagine this then makes their vision un-corrected.
ALL of this, is probably most certainly because of his eye problems. BUT simply ask the eye Doctor. That is the only way to get answers to this.
Also, is he wearing kid glasses... the kind with the strap on the back...? I know Sear's has MANY different kinds of kids glasses... even for swimming. And I'm sure many other opticians would have kids glasses too, with a strap on the back. That may help him to keep it on...?
In the meantime, he is 15 months old now, and has his next check-up at 18 months... but this does not mean you cannot take him in for an appointment before then. A child/baby can be taken to the Doctor anytime, as necessary.
By the way, it's a good Grandma you are, that you are "observant"... but without being petty about it. :)
All the best,
Susan