The protagonist was weak but now he is really powerful however he still looks weak. it's a martial arts manga and I think the protagonist uses muay Thai to fight. the latest chapter I can from it was that the protagonist was fighting this American guy because he was making fun of japan. thanks in advance. 4s2y2g

Sounds like Tiji-kun!.

no I think the main character decided to take muay thai because his love interest didn't like weak people so he wanted to become stronger

I don't an american in the whole Tiji-kun! manga, much less him insulting Japan.

You can browse these tags : Muay Thai and Kickboxing
Not much listed, so it shouldn't take you long.

Your description does remind me of Karate Shoukoushi Kohinata Minoru. It's about karate mainly, but the main learns from muay-thai too at some point for his kicks. It has the girlfriend looking up strong people, and the MC facing some american in a bar or something, where illegal fights happen.

Is it history's strongest disciple kenichi? That's the only karate one I know.

Is it Hakaiou Noritaka!?

Yeah this is it thanks