Sources close to Sugar Bear tell TMZ ... he reached out to Lamar over the weekend about going to one of his treatment centers, and on Thursday evening he will check into a facility in Kansas to focus on his recovery.

We're told Honey Boo Boo's dad is hooked on prescription drugs he started taking as part of his diabetes treatments, and he also has severe depression and anxiety.

Our sources say Sugar Bear first met Lamar last summer through their manager, and they've since established a friendship, which led him to start rehab at the Gold Bridge Treatment Center in Louisburg -- part of the Odom Recovery Group.

We're told Sugar Bear hasn't had a relationship with Honey Boo Boo for more than a year now, and he was noticeably absent from her graduation. It's unclear if their estrangement is tied to his addiction.

Our sources say Sugar Bear is still hoping he can mend fences with his daughter once he's completed his rehab stint.

As we reported, Sugar Bear underwent an astonishing makeover last year following a cancer scare, but since then he's hit a rough patch -- we're told his most recent girlfriend broke up with him a month ago, and he hasn't been handling it well.