C.C.
Big picture about scouting, I think it can be easy to forget that "advancement" is not mandatory. Like with so many things in life (other than school!).
The "prerequisites" you describe are for merit badges. And merit badges are only required for advancement to certain ranks in scouting. There is no prohibition against existing as a scout at the most basic level...a boy can stay at the "lowest" rank (with no merit badge required) until he ages out of scouting at 18.
It's like playing an instrument - a child can practice, can be in school band, can compete...or can just learn how to play a few songs for fun.
Related to mamazita's response below, give some thought to whether you want to advocate for your son's scouting experience to be more fun/doing stuff he enjoys or more challenging with requirements. Either way of scouting can be rewarding. (I say "advocate" because obviously there is a whole group of boys involved around your son...you should try to get a sense of the other boys' and parents' attitudes...it might be uncomfortable for your son to be surrounded by boys with a more competitive attitude about scouting if he does not want to be that way, competitive peer pressure can make advancement *seem* mandatory.)
ALSO: I would encourage you to think carefully before you "assist" your son with obtaining merit badges. Think about it the same way you would approach "assisting" your son with his homework.