Talk about his Mom
I lost my husband two years ago, and with four children (the youngest was in 2nd grade at the time) I've come to realize the importance of talking about him as much as possible.
Just 2 days after my husband died, my son had his 8th birthday, and his 2nd grade teacher threw him a huge party in the classroom. While I was at the funeral home finalizing the details, she was helping our little boy blow out candles and open gifts. It was a tremendous gift to our family, knowing that the teacher was allowing him to be a regular kid on his birthday. I owe her so much, she was a stronghold for him during that entire first year of loss.
Back to your question, children want to tell you stories, and they want you to tell them stories about their parent. Find out as much as you can about his mom. Retell those stories, and let him tell you his own versions. (Write them into a journal type book, and present it to him at the end of the school year)
Each time you share about his mom, use the gentlest, sweetest voice you have, and rest assured that brings a child comfort.
Let him bring a picture of his mom to share and keep with the classroom.
My sister-in-law used my husband's old shirts to make pillows for my children, and they LOVE that gift. I was fortunate enough to have an entire quilt made out of his old shirts.... maybe finding a seamstress to make the pillow using one of his moms' shirts, for the little boy would be a wonderful gift idea.
Find out what sort of soap/perfume was mom's favorite, and bring a bottle of it to the classroom, spritz the little boy's desk... it'll also comfort him.
If his mom was a collector (some women are) help him keep that tradition alive. E.g. If she collected sunflowers, incorporate that into an art project.
Holidays are especially difficult, just be sensitive to his loss and loving to his hurting spirit.
Let him cry. He is a little boy, and don't let anyone (adult or child) tell him, "Don't cry." That's not fair that he squelch his feelings because someone else doesn't feel comfortable with sadness.
He may laugh and play a lot sooner than some people think is appropriate, but his grief is in HIS time frame. One of my son's friends told him when they saw him playing soccer, "Don't you feel sad anymore?" It had been less than a month, and of course he was still sad, but he is still a little boy, and his nature had not changed from that of a little boy.
I think that little boy in your classroom is blessed to have you as his teacher. I can tell how senstive you are to him. God bless you for your kindness to him.