So the main benefit you are looking for is to build quality tendons and core muscles in your son?
If so he can get that with most non-contact sports. Basketball (or any team sports really) is really good for general fitness and musculoskeletal development too, plus skills like dexterity, co-ordination,team work and emotional development. I would say a team sport is better for a young persons development - physical and social. There are other pastimes like Gymnastics, Wrestling, activities like riding, kayaking, rock climbing, skating, skateboarding, bike riding or hey how about dance classes too (girls
)!
But...if
he wants to practice martial arts then I would say do not assume that one art is like this and another is like that. Wing Chun is an art designed for beating people up who get in your face. There are many branches and teaching styles, some are internally focused, some are balanced and some are external and only concerned with efficient self defence. Its as simple as that. Dont assume that you wont get punched in the face during sparring or not have to do any real exercise because no two schools are the same - but at the end of the day it is meant to be martial practice. Also relaxation is the key to success in any art, not just WC or Tai Chi or whatever.
WC at my school does offer a good exercise regime for kids (with them doing a lot of weapons, gymnastics etc) and adults but sadly no one can speak for WC totally as an art.
Good luck.
P.S. he is only 12 so maybe he will mature if you give him a chance. Check that his diet is good, his rest and sleep and not 24/7 console games and TV.