Like everyone else watching Boruto, we are all curious who Metal Lee's mother. So here are my theories. 1.) Metal Lee was born out of wood lock while Rock Lee was away on a mission in a different village and the mother didn't want the baby so Lee took him in. Why I think this is because it's just a possibility..lol. I don't think Tenten is his mother because she never showed that kind of interest in Rock Lee and I feel like she was in love with Neji. But since Neji died, she never bothered to find other love.
2.) Metal Lee is adopted. Since Lee never showed interest to other women besides Sakura when they were still young. I just thought he adopted because he wanted someone to carry on his legacy.
3.) He went to Orichimura for help and just like Mitsuki, Metal Lee was born. These are all just theories that I made up. I don't have evidence to back them up..lol