This depends, in part, on your age and his. He's most likely coming on to you. Since you didn't reciprocate in anyway, he's likely not going to do it again. If he does, you might want to have a conversation with him. Like, I don't like that or I do. That part of your body isn't considered an erogenous (sexual) area. It's one of the least so. So, unless he's in a position of power over you--like, he's a relative, or an older (several years) person, or a teacher, etc., it's hard to define as an assault. Technically, ANY touching that is unwanted is an assault, however. So, just be clear with him if anything similar happens again. On the other hand, I've coached young people and have patted or squeezed a shoulder in order to congratulate, encourage, or get attention necessary to instruct. It was rare, but appropriate touching. If it is a situation like this, that is observable by many people, then there may not be anything else intended, but gender difference might also be considered in this situation. A man who touches a boy or girl, whether or not it is in public, might be trying to manipulate that girl or boy into closer physical relations--and could be a sexual predator. Do not jump to conclusions, however. And yet, if you don't like the behavior, that's best expressed to the person, and if you're a minor, best expressed by someone who represents you-- that is, an adult of equal power or authority. If you're a young person, under say 25 years old, tell your parents or a person of authority you trust and can confide in. A woman who is a doctor (or if you trust your own male doctor, if he is male) is also a good person to consult.