My daughter is the same as your son, she's 14 months, I guess she just needs time to get herself ready. I'm not that worry as long as she can walk by 18 months.
My daughter is almost 13 months and still not yet walk by herself. she can stand without support for about 1 mins only. However, she walk very well if holding my finger or pushing a walker. So, I dont think she can be able to walk alone before 15 months.
Dont worry! you cannot do anything to force your baby walk.. just wait.. I beliver they will walk when the time is ready!