Can I get 3rd stimulus money for new dependent when ex claimed him last year

My ex-mother-in-law had my 16 year old last year for 7 months and claimed him on her 2020 taxes but he moved in with me in August 2020. I want to know how I  can get the 3rd stimulus and on for him. We really need the money right now I don’t wanna have to wait till the 2021 tax season