The testicles are extremely sensitive to pain and are not usually considered subject to erotic stimulation to any particular degree. Now lets go back to the shaft of the matter. The opening in the tip of the glans penis is the meatus. Here is where the cum spurts. There are other parts of a man's body which respond with alacrity to oral stimulation.
Many men are particularly sensitive around the nipples.
The first time I kissed my partner's nipples he shot before I had the chance to even get near his cock. So LOOK at your partner's penis. Learn its areas of special sensitivity completely and be ready to apply your knowledge to his body with your tongue and with your lips when you bend your loving head over his cock.