Well, you will not stretch lengthwise. Ever, so don't try. Ouch!!!
And you should really ask him if he would like more stimulation to his base. The most sensitive part of a man's penis is the tip (much like a vagina) so he might actually not care. Talk to him about it.
However, there are penis rings (sorry for the language, but I don't know them by any other name) which traditionally are to strengthen an erection. But now they have larger softer (silicone or whatever) ones that also have a vibrating aspect. You push it down to the base of the penis, turn it on, and it vibrates, which gives both of you very nice sensations (most even have a part that stimulates the clit at the same time). So he would feel the gentle vibration on the bottom of his penis, and you would also feel the vibration on your clit.
You should be able to find them online or any place that sells sex toys.
That's the best thing I can think of.