I'd celebrate at the same time, we've combined many celebrations over the years. My guy's birthday is next week, his party is on the 13th. We're having his whole thing, plus a lemon cake in honor of my Mom's birthday, the 8th, she passed last November. Plus, my sister's MIL, she and our Mom share the same birthday, PLUS my 11 year old nephew earned his black belt in Tae Kwon Do last Saturday, we're celebrating that, too :)
I wouldn't think their feelings should be hurt, I would say since they live aways you'd like to make it easier for everyone and celebrate his birthday and Mother's Day together, have a special dessert for each person, make it a big deal. BTW, my grandson's first birthday party was at my wedding reception!