I need help telling boss about pregnancy

I currently am a bookkeeper/ personal assistant. Lately my boss has become very needy (job has recently turned into personal assisting) and has made upsetting comments before saying I need to let him know before I'm even planning on getting pregnant. I'm now 15 weeks and have no idea how to let him know. My job offers no maternity leave and fear that any time off will result in me losing my job. I would love some advice on how to break it to him and keep my job secure