IMO/E....9 weeks is near adult...I have spawned at that age...
not recommended.....but some/can spawn...really depending on goals...But for general keeping a 2 month old should be about or at least nearing adult body size-it can take a bit longer on the fin length/development-this depends on tail/fin type too....The long fin males can take 4 months and the HM I like to wait 6 months to make sure they can hold/carry their fin/tail...anyway....
I generally stop feeding BBS to my fry at 8-12 weeks of age-depending on growth/development-But regardless-by 8-9 weeks they are on a different diet-I have found the BBS are too small for them to eat anyway-I may supplement with BBS- I am weaning them from the live foods by that point so that they will know how to eat processed foods-plus-I want a heartier diet than what BBS can provide-along with my homemade food-I will add processed food, mosquito larva and some HUFA supplemented brine shrimp to get best growth/development.
As for the amount to feed....BBS should be more of a snack at this age...hard to give amount since the BBS can be different in size...but roughly 50-100 BBS...more or less- along with some processed food and/or mosquito larva-Watch the tummy....you want it to be nice and round/full looking and depending on the Betta color you may or may not be able to see the orange color of the BBS in the stomach
Post a pic of him along side a common item to compare him with....might be able to tell or give estimate on the age....but growth/development can vary as a general rule based on a lot of different factors...
Did you buy him from a breeder or did you buy one of the baby Betta from a pet shop