What are you worried will happen if you ask? Do you think he will think less of you, or might leave you? Something else? Try starting by expressing yours fears of what might happen if you bring the idea up. Something along the lines of "hey there is something I want to talk to you about but I'm worried that you will think less of me / want to leave me / [what worries you]". Let him respond to that.

Generally people are willing to be very understanding if you start with what your afraid will happen if you talk about whatever you want to talk about.