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.

Agree with Cindy. Every baby develops at a different rate. I have a friend whose boy was similar to your child, refusing to stand but seemed to have strong leg muscles (he does "bicycle kicks" often when he is lying on his back). My friend consulted her PD and PD told her not to worry and just wait a couple months more. He eventually started trying to stand after he was 13 months old. I would suggest speaking to your PD, you can at least get some suggestions on how you could help strength his muscles or encourage him to stand/bear his own weight.