Lamar Odom's Memory of Son's Death Pushed Him to Rehab

Lamar Odom checked himself into rehab out of fear he'd go on a major bender as his deceased son's birthday approached ... TMZ Sports has learned.

Lamar's son, Jayden, was born Dec 15, 2005, but died the following June from SIDS. We're told his son's death was the primary reason he started doing hardcore drugs ... and he was keenly aware the birthday might push him over the edge again.

As we reported, our Lamar sources say he went to a San Diego area facility late last week.

We're told Lamar's also struggling with the fact his divorce from Khloe will be final on Saturday, and he didn't want to be tempted to use.

Lamar is shooting for a 60 day rehab stint. He bailed after 3 weeks when he attempted rehab in 2012.