After having chatting about sex toys for men with my boyfriend, he mentioned how he would love a toy that either straddles the balls and gives vibrations or one that sucks the balls. I am yet to come across a toy designed for the testicles and wondered whether there are many men who particularly want more attention to them...
You could buy a stretchy cock ring with a vibrating bullet on it - we got one of the durex play ones and that was stretchy enough to go around the shaft of my man's penis and his balls - I positioned the bullet at the back resting on his balls and it was fantastic, he actually whimpered ;)
Not sure if there's anything that does what your boyfriend actually wants though, but that may be worth a try.
I agree with Private Assistant, vibrating cock ring swivelled around so it's on/under his balls and not on top of his cock. Should give him a bit of a thrill!
Which Durex ring were you doing it with? The newish Durex Touch - http://www.lovehoney.co.uk/product.cfm?p=13257
Or the new version of their Vibrations ring? http://www.lovehoney.co.uk/product.cfm?p=3542
The LoveHoney Bada Bing Ring would probably be a good option too - it's chunkier than most and also has touch-sensitive vibrations, so you can make him whimper at your whim ;-)