It's normal to feel a little depressed about losing a loved one especially a mother and grandmother that practically raised him and also especially around Mother's Day, just try not to leave him alone too much go to the Cemetery with him just tell him you are trying to be supportive of him and that you don't want him to feel alone this way he will know that even though he lost his loved ones he still has someone that truly cares and loves him. Support his decision to go to the grave site. Don't let him be alone too much during this time it may pass when Mother's Day passes but if it doesn't maybe suggest counseling for him, tell him it just helps to talk to someone about his loss and to learn to cope with the loss.
There are many people that go to their mothers and grandmothers grave site on special days like Mother's Day and birthdays don't discourage him this will make him withdraw even more but show him that you support his decision to honor his mother and grandmother during this time. You could even buy him the flowers to put on the grave he may appreciate your gesture and it could help him to open up to you more because he sees that you do understand how important this is to him. If you still need more help let me know.