My 9 month old is small for his age?

He turns 9 months in two days. I took him to the doctor last week because he was pulling on his ear and I thought he had an ear infection. I turned out he was just teething. When they weighed him, he was only 17.2 lbs. I asked the doctor if he was under weight, and he is said he is in the small part of normal. I am a bit worried, he is fed and on solid food. How much does your 9 month old weigh. I plan to aks more question when he has his 9 month check up on Friday. Also if you think of any questions I should ask, feel free to post them. thanks