Hi - I have an 8 1/2yr old son - he is skinny and used to cry as though he had serious injuries with scraped knee, etc.
My husband and mother-in-law are always trying to tell him to eat so he will put on muscles. However according to the pediatrician ( and I fully agree with Cherryjam below) , he is TOO YOUNG.
These boys are skinny because they are boys. He will develop them when puberty really kicks in and as he develops into a man.
Also from your note, it sounds like he is your oldest child? Typically 1st borns are more insecure and those 2nd children are more secure/confident/outgoing.
Having him in karate is a great thing. It will help his character if you have a karate studio with instructors who are tough but caring with the kids . ( We do. )
Also, if he is a 'gifted' child, they tend to be more sensitive and experience things more intensely. See books from Dr. James Webb.
Remember though - this sensitive boy, may be mr rough and tough in a few years, too cool to come to mom for anything.
As he hangs out with dad/male role models playing sports, doing things, they can help me develop a more non-chalant - oh, I got hurt, is it that bad,…hmmm no, I can live attitude.
Our son has grown in this area, where the crying hurt time has diminished considerably and will continue to do so as he gets older. Hug him, let him know he's loved, after I have tended to the injury - if it needs it, I then joke around with my son:'Oh my , is it so bad we need to call the doctor to remove your leg?' 'Or should we put you in bed all day - no more playing?', stuff like that because we joke around a lot in our family and it helps him go:'Come on , Mom.' and roll the eyes at me instead of being so upset over his injury.
Be in tune with: has he been tired, rough day the day before, things that might be adding to why a minor injury is so upsetting to him.
Love and hugs to you! Hope this helps!