D.B.
Kids often express more affection and say "I love you" more often to a person other than the mother. They sort of take Mom for granted, and they feel that the love is understood. It's an unbreakable bond and they often don't feel the need to reinforce it because they think it's a "given".
Working moms never feel they have enough time for their children. Try not to feel guilty - I know it's difficult. But I don't think that's why he's not saying it. My son didn't say it much either. He was much more open with my husband.
Just keep reminding him that you love him. You can comment when he says he loves Grandma that "It's so nice for you to say that. Everyone needs to hear that they are loved. You like it when Grandma and I tell you we love you, right?" But don't nag him and say "Why don't you tell me you love me? I'm sad." You have to stop short of that, because otherwise the children clam up or they are just saying it because you've "demanded" it, and that doesn't feel good to the parent! It's not the child's job to keep the mom happy.