Bam Margera Says He's Pursuing Sobriety to Secure Relationship with Son

Bam Margera says he's working hard on sobering up and staying clean ... all in the hopes he can be reunited with his son and maintain a relationship with him.

In new court docs, obtained by TMZ, Bam says he's currently sober, and is actively working through his struggles to better himself.

As a result, he's requesting joint legal and physical custody of Phoenix -- asking for things like overnight stays for the 5-year-old every other weekend -- so he can forge a better bond with his son.

Bam says he's surprised Nikki, his estranged wife, is unwilling to support him on his journey to sobriety, because he claims she's also struggled with sobriety in the past. In his defense, he adds .. he's already gotten treatment from multiple resources, completed courses and started therapy.

The former "Jackass" star claims Nikki's doing everything she can to keep Phoenix away from him, even allegedly refusing to let them talk on the phone anymore ... and says she's also isolating their son from Bam's extended family.

As we reported, Bam and Nikki were at an L.A. court this month to, in part, establish if their 2013 marriage was valid -- Bam believes their wedding in Iceland wasn't an official ceremony, because they weren't residents there.