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.

Every kid is different. You as a parent you are different too. Don't stress. What you want is to let baby have its space to do its own things sooner or later you will wish they could just stay put at a place. If you really want to work out the baby. Try swimming. Teach them how to kick