super easy I mean us guys can get turned on at any time over anything especially someone we're are attracted to. usually any type of contact from someone else aka you will give us a boner. for example just making out most guys will get a boner sum times even just holding hands or sitting in his lap will turn him on.just being able to touch your body in his mind is a turn on and very physically sexy so yes we can get turned one very easily especially during our teens and twentys. many guys deny getting a boner just from grinding but trush is every guys gets hard unless there's a major turnoff that he's thinking about. hope this helps
He's been aroused before (from you) and is probably nervous / scared to pursue anything sexual with you. So feeling your body is his way of imagining how awesome the sensations will be in the future. Woman that are extremely attractive only need to walk into a room for many guys to become aroused.
Really easy just seeing or thinking about a girl can turn us on. Does he get a boner if you do that, we can get one of those just dancing with a girl. If he has liked you a long time even hand holding can be erotic. Be cautious. If you don't want to have sex(he does from your description) let him know long before there is too much touching he will probably accept that but it can get awkward because guys are sort of like a train when we get turned on its not easy to put on the brakes
case in point: my boyfriend and I were talking on the phone as he was on his way home because he was working late one night. he asked what I was wearing and I told him ending with me telling I had on my black g-string. next thing I knew, he was on my doorstep. lol