How long has your agent had your novel? What do the agents guidelines say? If he's had it longer than 6 weeks I don't see any harm in sending him a polite, brief email asking when you might expect to hear from him. After all, you do have a previous and current relationship with him. If less than 6 weeks, you might want to wait the "standard" 6 to 8 weeks before inquiring about your work. I think the question begs knowledge about your relationship with the agent.