Car Foreplay??

So I have my boyfriend he's 21 and I'm 18 and he does this thing where when we are sitting in the car and he leans over to kiss me and he puts his hand in my pants and makes my whole body and shake... but he never wants anything in return I feel weird when he does it. I wonder if maybe my moans sound weird and stuff... How do I ask him about it without making it awkward.