My Family Reunion

So my boyfriend of 4 months wants to come to my family reunion and I'd want him to come and meet my family too. However, recently my granddad passed away and my entire family's grieving his loss right now. The day after my family reunion is his memorial service. I asked my mom if he could still go or not and she said no because it's probably not the best time. I told my boyfriend that and now he's really upset because he just wants to be there for me and meet my family and now he's starting to get the wrong idea of my mom and now he's starting to think that she hates him or something. We both feel as though he's a part of my family but right now, with our family situation, my mom just doesn't think it's a good time and I don't know what else to tell him. Should ask my mom for more details about her answer or should I just leave it at bay and let my boyfriend continue to feel uncertain about me and my mom's judgement of him?