What do I do if one of my students is behaving rudely and ignoring me?

Have you tried taking him aside after everyone else has been given something to do and asking him what's up? 14-year-olds are moody, hormonal, and angst-ridden and he's probably just being a typically teen, but if you talk to him calmly and ask him why he seems so distracted maybe he'll be more open to working with you and participating.