You mean turn him into a walking nuke that could obliterate everything if he isn't careful in using it which is already hard considering that using the form causes him to go berserk..

I'm fine with that

What about when he gets all the forms before final form, something snaps inside of him? Something "Hulk" drives him insane destroying everything in its path while killing its enemies.

Paleo.... SMASH!!!!

If there would also be a Tarzan vibe, not the savage wild man look, but when he does the shout....

https://www.youtube.com/watch?v=hEzJJe3RYe8
https://www.youtube.com/watch?v=WiAQmImx7nM

.... after he defeats an enemy kaijin. Perhaps something half-dinosaur starts to fuel inside of him.

Nah its better if the final form itself is the one messing his head because I would love to see him struggle as the situation pressures him to use it while knowing the consequences and difficulty and just say

" how the hell am I supposed to use this without destroying the city!!"

I had an idea for a secondary rider; one based on the early, early shell creatures. I know we should focus on paleo, but it's just a suggestion.

I like that maybe the side character could be his whole reason behind being able to control it all..Oh yeah I'd love to be on the story team btw

I'll stick to the secondary based on ice age beasts.

Okay, maybe a tertiary rider?

I was thinking that instead of the usual tomodachi powers, he must remove all doubt in his mind and focus on his goal, you know...clear heart and clear mind since the form messes with them.

Then let's yank the chain some more that after mastering the form, some of his friends and the city people start to fear him and see him as a monster after witnessing what the form could do, thus putting more doubt in his mind and cause him to lose control a again which ends up with the people fearing him even more until they finally deem him as an even bigger threat to humanity than the monsters themselves and some even teaming up with them to get rid of Paleo while the the poor guy desperately tries to prevent himself from breaking emotionally and mentally from the stress.

..it is called the Extinction form for a reason.

Cue Devilman ending for ultimate bad end.

I did say could get dark when I write it, heck breaking cuties is my specialty..although I did go too far sometimes