I usually dissuade P from touching the toilet. I am getting into the habit of sitting her on it every now and again with lots of praise etc, but I don't allow her to touch it because before I know it she'd be climbing in there and I couldn't cope with that.
What I would do is, if he reaches out to touch it, I'd be (happily) saying, "we don't touch it, we sit on it" and put him on, even just for half a second. Fact is, toilets are dirty and they're for sitting on, not for touching/playing with.
I think it is a jolly good idea to let him see you on it though. Although I am yet to come across any parent who can escape their toddler long enough to go to the toilet in peace sans audience! LOL.