My wife said she heard that could make him bow-legged - so I looked it up, and found a thread full of people saying that's an old wive's tale.
Four years ago my mother-in-law said the same thing, I knew it was not true but to make my husband relax I asked the pediatrician. He told us that it is an old wives tale from back when babies were on pablum and not eating any fruit, because it is the vitamin deficiency that caused babies in those days to get bow-legs. So the answer is that it is not true. My son is 4 and straight- legged, and he was an early assisted stander.
Sounds reasonable. So, he'll keep on getting this kind of exercise! I also wish I could find more recommended exercises/games for infants; I understand spending time on his stomach is probably important, but didn't find many other recommendations.