Lamar Odom's father says he's NOT gravely ill ... and was only suffering from a stomach virus earlier this week ... but he's grateful that his famous son would skip an NBA game to be by his side.

Joe Odom was out in L.A. yesterday and explained, "[Wednesday] night [Lamar] stayed with me all night ... just kickin' it and whatnot ... but everything is good now I feel a lot better."

The Mavs were scheduled to play the Lakers that night (L.O.'s former team), but Lamar was excused for what the team called "a family matter."