Hi, first of all I’m Spanish so if you need help you have my messages open! If you wanna say it as a Spanish one, you’d say
El chico escaló la montaña, if you want to be a little more informal but also Spanish, el chaval escaló la montaña, also if you don’t wanna use the verb escaló you can use subió, is not the same but some people in Spain say it like that, and if you wanna say it like a Mexican/ Argentinian/ Colombian......
El chico trepó la montaña, also if you wanna be a bit more informal but latin too you’d say, el chavo trepó la montaña