i think you need to be careful when using terms like mentally challenged - this includes a wide spectrum of people from those with autism and aspergers who have a different mental make up but may have a huge IQ, to those whom we used to call 'simple' which I presume is what you are talking about.
if you want to engage with a person on more than a social friendly basis then you really need to be sure that you're not taking advantage and reading signals wrong and that the person involved truly understands the implications.
If however you're merely saying he has a low iq (judged how? ) but he is mentally aware thats fine - but don't associate low IQ with lack of intelligence which comes in many forms - and high IQ doesn't equate to intelligence either (we've enough daily proof of that with our politicians and the likes of Lehman brothers).
So you do need to phrase your question more carefully i think, and understand the person you're talking about, but so long as you're not taking any advantage of that person and anything you do is in a loving/caring/respectful way, I personally think thats ok.