I can understand why this would be so bothering to you. Little kids cry, but not all the time. Even if he did have ADHD, it would not cause the crying as far as I know. I would suggest having him checked out by your doctor. Check his weight, etc. Does he have the normal toys that kids of his age have? And things like swings, stuffed animals, .... things to occupy him?
I think there must be something upsetting him I doubt he has mental retardation...How is his health, is he eating well, does he poop okay ..has he siblings, does he go to daycare .What happens prior to a crying spell ? sorry for the questions we need to know mopre about him ..