To talk with him, just get his attention by touching his legs or the newspaper he's always reading. Once you get him to put down the paper, you can start a conversation where Ben will repeat everything you say to him. You can even save the conversations and send them to your friends, as it's pretty funny to hear yourself talking in a dog-voice.
Ben will also teach you some chemistry in his lab, where he keeps many strange substances that you can combine to discover different results. If you're lucky, Ben will be able to play with your creations without them exploding.
If you want to keep your pet happy, you'll have to feed him and give him water, which can be especially useful and educational with small children. In fact there is a special mode for this purpose, in which they are encouraged to take care of Ben the Dog and keep him well-fed at all times.