I dont really have an answer for you, but it might be hard to find others who are exclusively giving breast milk at 9 months. Are you doing solids as well? If not, is there a reason from your doctor? The goal I was given by my doctor was to start solids at 6 months for iron intake with 1 meal a day, and slowly increase the amount of solids to 3 meals a day plus snacks by 9 months. Their milk intake will gradually decrease over this time as well to be more complementary to the food instead of the other way around from 6-9 months. Milk might only be a few times a day after naps by 9 months.

Anyway, to answer your question about weight, I would just put his weight today vs his weight a while ago into a baby weight percentile calculator and see if he's still growing on his percentile curve or if he's dropping percentiles. That will give you a better idea of your son's growth against whats "normal"