Yes, a child continuously slapping another could be due to attention seeking. It can also be due to anger and frustration. A child being ridiculed by peer group often lashes out at others. A child can also have violent behavior as he is hyperactive and needs an activity. These children can be managed well by keeping them fruitfully occupied and lavishing praise and love on them. Take care!
This child could also be stimming. Children/people with autism often hit when they stim. It is a way to self soothe. The best thing that can be done is to get him some occupational therapy. NEVER praise stimming, but don't discourage it if it doesn't harm anyone.
When he slaps the other children, tell him no this is not ok. Put him in time out if you can or take away a privilege. Be consistent and punish him every time he does it. If he can talk have him apologize to the other child. Even if he is stimming this is a good way to handle it. Giving him praise and attention when he's acting like this will only reinforce the behavior.