I really wouldn't be worried about it. Most students apply for several positions and then choose one. Tell him/her that you found his offer very interesting, thak him/her for the plesant interview (if it was), but you now have accepted a different PhD poistion that suits u better.
But do it as soon as posibble- he/she won't be angry if you don't take the position- but she/he will definatly be angry if you don't tell them, and they then can't offer it to someone else.