The Cleveland Show/WMG

Cleveland Jr. got a lobotomy sometime before the show.
What else could explain the massive change in character? Also, part of his hypothalamus got damaged in the process, explaining his current girth.
 * Alternatively, the second-to-last mishap to befall the Browns at Peter's hands gave Cleveland Jr. permanent brain damage. Which is why the last one pissed Cleveland Sr. off so much.
 * Clevelend Jr. could have gotten on A.D.D medication that caused him to mellow out. The fact that he was no longer engaging in so much physical activity caused him to gain weight.
 * Or, shades of Principal Skinner, the real Cleveland Junior wandered off in one of his hyperactive phases while in Loretta's custody. Desperate to not screw up her undeserved divorce agreement, she found the overweight boy we see, and 'Uh-Huh!' insisted the weak-willed Cleveland into believing this was their son. In a further twist, the overweight CJ will turn out to be the illegitimate son of... Cleveland and Donna, ditched after a 'final fling' about the same time the hyperactive CJ was born... so yeah.

Cleveland Jr. got taken by the wolves between his last appearance on Family Guy and The Cleveland Show.
And thus, he has been left horribly roont.

Holt is a female-to-male Transsexual.
He's short, overly macho, has issues with his mother that reminds one of mother/daughter issues, and has shit luck with women, having little to no confidence in himself and. Of course, he's post-op, but his insecurities still hang over him -- hence the macho behavior.
 * That's... kinda insulting to actual transgendered folks.
 * Its a spin off of Family Guy, you expect them to be properly represented?
 * Plus, well, the original author of this WMG is trans himself.
 * The creator of the preceding reply to this WMG found N Word Privileges to be hilarious.

After Loretta cheated on him, Cleveland moves in with the Griffins.
And goes a bit bugnuts. This explains why so many of the characters are similar. Erudite talking animal? Young rage-crazed, talking toddler? Disabled friend? The Griffins, in their blaise, just don't notice Cleveland has developed an insane fantasy life.

Robert Tubbs is the bastard son of LeVar "Freight Train" Brown.
They're too much alike

Rollo is Gay.
And he is trying to hide it.
 * Kind of like Stewie in Family Guy?
 * Doubtful, so far he hasn't even question his sexuality.

Cleveland Jr. doesn't believe in God because...
He lost faith in him following his parents' divorce.
 * Alternate theory: He prayed for God to make him thin again after, eating the equivelant of two of his former selves, and after not getting his wish he proved it to himself that God wasn't real.
 * Junior should know that God performs miracles, not wishes.

Cleveland Jr. in the show is from another universe.
The first episode of Family Guy to air after this show's debut was the multiverse episode, and it even shows Cleveland still living in Quahog in the Disney Universe. It's possible that the Cleveland Jr. we see is from a universe where people are fat, and he escaped with Stewie.
 * This may be jossed in the golf episode. Such episode includes Cleveland Jr mimicking himself when he played golf in Quahog. He admitted to himself that he gained a few pounds which implies he just gained weight.