You should never been in a Martial arts situation without a cup.
Even when Im just at home with my brother, doing some Thai pad work, or non-contact or light contact drills, I always wear a cup. I use a Twins Steel cup, made in Thailand, designed for use by Thai boxers. For BJJ, Karate and Judo I wouldnt say you need a steel one, a strong plastic one would do, since the groin shots you will take there arent full on blasting shots, they are more like glancing blows and accidental bumps (which as Im sure you know, can be amazingly painful in themselves!).
That said, I think the Twins steel cup is so comfortable, that I could do Judo with it on and hardly even notice its there, so you might want to get a steel one, just to be on the safe side. Make sure you get the right size cup though, if you get one and it doesnt feel perfect, send it back and get another size. If you don't get the right size it will feel uncomfortable, it will either squash your nuts, or slip around during training.
The worst thing that can happen from not having a properly fitting cup, is that one of your testicles slips out of it, and if you take a shot while one is hanging out, then it can destroy the vas defrans (sperm duct) and the blood vessals that supply the testicle, yes, you can lose the testicle! Don't get too worried about that though, if you have a properly fitting cup the chances of it happening are nearly nil.
So in summary, a cup is essential imo, you might think "Oh doing this drill or exercise, there is no way Im taking a shot in the balls" - its always the shot you dont expect to take that gets you!
One last thing; getting hit in the nuts while wearing a cup will still hurt! You will still get cramps, feel sick, get clammy and go pale etc, cups dont stop the pain, but they do stop damage to one of your most essential, and vulnerable organs!
Buy a cup, and one for your son too!