You might want to post this in the "Hobbies/cycling" section and maybe get more answers.
I don't know how tall your son is, so you might want to go to a bicycle store and get your son measured to see if 20" is appropriate for him. Also look into the weight and see if your 8 yr old son can handle the bike.
But, just looking back when I was 8 yrs old, if I was given this for my Bday, I would be jumping with joy