There is so many skills he could train.
He should pratice anything he can access.
Starting with a fast reading skill then devouring any tutorial book available.
Also, going to a local gym and work on anything they have.
Shared universe : https://mangak.io/lists/QDwlzEe2-martial-gods-inn
Medical skills series : https://mangak.io/lists/bDManO0D-medical
More lists on my profile
Medical skills series : https://mangak.io/lists/bDManO0D-medical
More lists on my profile