I know the other guys have already mentioned SSRIs. They really are sh!t for everyone involved (and unless someone is in the most serious state of depression, ie in hospital and even getting dressed is far too difficult for them, then SSRIs are no more use than a placebo Dead Link Removed If either of you are on them, read the link and then ask yourselves why? If you do decide to come off them, careful to come off slowly - from what I've seen SSRI cold turkey is more hellish than from heroin).
OK, stimulation, are you using condoms? they can really cut down stimulation and sensation.
has he been mutilated? over half of the sense receptors that a male was meant to have, were in the inside of the foreskin and the frenulum, if it is gone, they are gone.
What can you do to increase stimulation?
Do it face to face with the lights on, so he can see your face and your enjoyment, you can communicate, kiss etc.
don't wear a T shirt or something like, which lessens skin on skin contact with him.
Stimulate some of his other sensitive areas at the same time, nipples - rub them, tweak them, nibble them, - both at once is even better than just one. if he'll let you, and you don't have sharp fingernails, gently go for his anus too, just inside at the front you can stimulate his prostate. The more you can stimulate at the same time, the better.
Happy experimenting.