Do you mean stand alone, or stand with support? Before you panic remember there is a large age range for babies to meet their milestones. Is your baby able to support himself for short periods of time while you hold his upper torso? Standing alone or with support could still be considered within normal range. I would be concerned if your baby could not put any weight on his/her legs.
What does your doctor say? At babies doctor visits they should be checking to see if your baby is meeting his milestones.
You can ask your doctor for a referral to have your baby evaluated by "Help me Grow" or similar program. They would be able to identify if there is a need for early intervention or if its not something to worry about. They can provide therapies if needed. They can give you ideas to strengthen babies legs, etc. even if services aren't needed.
It is hard when you get the idea that your baby might not be developing at the "normal" rate. Especially when you finally get a moment to breathe and turn to the internet for information. Then you end up terrified because of all of the different recommendations for the milestones. You must absolutely talk to the pediatrician about this. Don't EVER let anyone at the doctor's office make you think that a question is stupid or should not be asked. If it is a question about your baby's health, you MUST ask it. Your baby cannot advocate for himself, so you must do it for him Even if other people don't like the questions you ask. That is their problem, not yours!
If you are in the US, most states have programs to help figure out if a child is meeting the milestones appropriately. Many of them also have programs to help children who are behind in development and these programs are usually FREE. The earlier a problem is noticed, the better the long term outcome. With my oldest child we did not notice his problems until he was about 4. With his little brother we noticed similar issues well before he was a year old. The very early interventions made a HUGE impact on him, and on how his brain developed. My older son has a harder time with a lot of things, but he did stop being a difficult child and now he is a self supporting adult who we thoroughly enjoy.
As a mother, you have very powerful instincts about your child. These are there to ensure that your child grows up and is successful in an evolutionary sense. The times I made the biggest mistakes with my kids are all times I let someone convince me to not follow my instincts. I knew, on a deep down level, that something was wrong with whatever was going on. ALWAYS follow your instincts until you get the answers you need. If that gut feeling tells you something is not right, or is very wrong, then it probably is. Keep asking questions until you get an answer that makes sense and is helpful. For the most part, your instincts won't steer you wrong.
So if something in you thinks something is wrong with the baby, get a doctor's appointment. Contact your local Health Department and/or Department of Human Services and ask what screenings they do on babies. Ask if they have any programs that could help your baby?
Most babies eventually make their milestones. Sometimes if a child lags behind for a while, they then catch up and move to the next one faster than is typical. My younger son was a late crawler. And a late walker. It bothered me a LOT. At the doctor's advice, I started paying close attention to his interactions with his big brother and sister. Youngest didn't HAVE to crawl, or walk, or even really speak to get what he needed and wanted. His big sister (4 yrs older) thought he was "her" baby. Anything it looked like he wanted, she would get for him or do for him. He would sit there and point and she would get whatever it was. Or he would make noises that she understood to be "he wants to go over there now" and she would carry him there. Once I got my daughter's behavior sorted out a bit, Youngest caught up to all of those milestones in just a couple of months. He finally HAD to crawl or walk or climb to get the cookies or toys or whatever.
That is a pretty simple example, but it was one that I worried about. I know how hard it is to sit on your computer as you worry and find all kinds of information that may or may not be the truth. Reaching out for help in a forum can be good, but it cannot be a substitute for a medical professional who knows how to advise you on your specific child.
My son didn't walk until 15 months. He is fine on every level of cognitive development. I think he didnt stand until 10 months. He is all grown up now with no developmental issues. There are no magic times. All kids do things at their own paces. If he seems alert and interactive, dont worry