If he shows a little bit of interest in piano, I say go for a keyboard. You can change instrument sounds so it will keep it interesting AND...they usually have a headphone jack
This is a great way for a child to start. The piano is one of the best ways to understand how music works and it will help him in the long run if we wants to pursue music to a grander scale.
Definitely let him take it at his own pace. It needs to remain fun in order for him to stick to it. If it becomes work, he will likely lose interest.
Pay close attention to his progress. If it turns out that piano/keyboard is not for him, have him try something else. Be supportive of his skills, let him know he's doing well and that you're proud of him. Encouragement is such a positive influence.
How exciting! If I ever have kids, I hope they express an interest in music
Good luck!