How do i tell him "i love him?" he has a gf!!?

ok ive know this guy named ryan for about 4 years,hes been going wth his gf for 2 years.i feel like hes growing on me and we spend tons of time together more than he does with his gf.even though he has a gf i think he likes me but i love him.i dont know what to do!i know lifes about taking risks and just going for what you belive in but i dont know what to do when it comes to telling guys how i feel.i mean ive know him for a long time which i think is the problem,because what if i tell him how i feel and he doesnt feel the same way?it could seriosly ruin our freindship!!please help me!!