How do you mean he starts "picking on him"  Is it in a way to encourage his little brother to play with him??  I find with my two boys 7 and 3 yrs they annoy each other as a way of trying to play with them.  It's a way of getting attention too.  My oldest boy does it out of boredom too.  So perhaps if your oldest boy is bored he'll pick on his brother.

Perhaps remove your oldest boy from where your youngest is, and get him involved in something else, reading or drawing etc.  Or get him to help you if you're doing something like baking/cooking, or even housework.  That's also a time you can spend with your oldest and he'll appreciate that.  Hope that helps.