Should it be a concern if my baby is 8m old and doesn't bear any weight on his legs at all? His legs will bent if we try to put him on standing position. Most babies seem to be able to do that by this age.

I don't think it should be a cause for concern as babies develop at their own pace. Some are able to do it and some may take some time. But eventually, they will accomplish this milestone. You can also encourage her to stand more by letting her sit somewhere where she can grab and pull things like perhaps in front of her crib. Aside from the tummy time, this "stand up time" with support should be able to help too.