My sons father filed for joint custody I want full I filed first a year ago but then gave him the chance to be a good father he got arrested with my son in the car but somehow was not charged with child in dangerment! Now a year later he filed for 50/50 I need to get full custody of my 4yr old that has a health condition (hemophilia) I need to as well prove he is not finatially stable and he sells marijuana but I don't know how to prove it what do I do??

You have to PROVE he's a danger to the child using EVIDENCE.

If he was arrested with your son in the car there will be a Police Report; I do not know that hemophilia would enter into custody nor do I know if it should; financial stability may or may not enter into things; if he deals drugs, that's important to PROVE.

You know the circumstances and I do not so you know what is available to you and what is not.

... i need to as well prove he is not finatially stable and he sells marijuana but i dont know how to prove it what do i do???

You prove it by your testimony. If

you saw him sell weed, or
if you know instanced of financial instability (whatever you mean by that), or
if he admitted to you specific examples of either or both of the above;

The evidence would be admitted. The judge can decide whether to believe you. How persuasive these facts will be toward the big question of custody is another issue.