I am uncircumcised and have gone down on a circumcised guy before. There is really no difference. It is best if he washes before if there is any smell. I almost never have a smell under my foreskin because I shower every day. If his foreskin does not go back on erection, then you could pull it back. Also you should be genital because uncut guys can be more sensitive. Personally I prefer uncut guys and none of my girlfriends never complained.

The nerve endings in the glans of a natural male are much more sensitive. This can be a good thing, just that you have to be a bit more receptive to his reactions at being touched. The foreskin is also well endowed with sensitive nerve endings so you have two areas that will provide supreme pleasure.

As Mr. Saint stated, I'd make sure he bathed/washed recently, although, this goes for anybody on the receiving end of oral stimulation.

Sensitivity and receptivity can vary from person to person. Some men may not want the glans stimulated directly, others will welcome it. Why the difference? Because the nerve endings of a circumcised penis have had years to desensitize themselves from exposure and having been in constant contact with clothing. As good as oral stimulation is for them, the sensations can be much more intense for the those intact individuals. You can stimulate him through the foreskin and/or if it will retract, stimulate both. See what works for him and what he can tolerate and go for it.

xbx, I can understand where you are coming from. I actually have never been with a guy who has not been circumcised. I dated a guy who had not been yet we broke up before it reached an intimate level. For me I see it as the land of the unknown.
From what I have read you need to be more gentle with the head of the uncircumcised penis. Perhaps the first time or two he will have to be a little more active than if he was circumcised. Perhaps have him show you how to pull back his foreskin, until you are comfortable doing it by yourself.
And yes because of this I would hope that he is a clean man. There was someone on here awhile back who wasn't as conscience of his personal washing as he should of been and was concerned because his foreskin being tight and not pulling it back as he thought it should.
If you like performing oral, as I myself do, then go for it. Take your time, explain to him that this is new and you would like him to be a more active participant. The only way that you're going to be more comfortable is to practice. If it's not what you want after trying it a time or two then I'm sure you will find other activities.

How Do I Get Him/Her to Orgasm From a Hand/Blow Job?