He was a weak man, bullied all his life for his size, and found security with Elvis and the MM.
Lamar was Elvis' punching bag: Elvis treated him like crap a lot of the time, but he came back for more. Elvis even had to pay to get him laid, in Germany.
If you watch the TTWII and On Tour outtakes, you can see Lamar's crest fall every time Elvis big noted himself by publicly making fun of Lamar. There's a point in every relationship where that sort of behaviour needs to stop, and Elvis often took it too far.
The On Tour outtake where Elvis pours a glass of water over Lamar despite Lamar seriously and honestly asking Elvis not to embarrass him is particulary sad. I don't like Elvis Presley at that moment.
Lamar probably should have said something, but maybe he was scared of being perceived as a wuss for saying his feelings were hurt? Besides, where else was he going to go?