I am truly sorry for your loss of your grandson. My daughter died at age 23, also, but they couldn't find a cause, so I know the pain you are feeling. I think a T-shirt quilt would be a wonderful way to keep him close. I've never made one, but my husband has been saving his to use like this, so I've gotten a couple of books on the subject. I've heard you should use fusible tricot (knit)stabilizer, because it would have the same kind of give as T-shirt fabric. To get the best stabilizing, fuse the tricot with the stretch going the opposite of the shirt's stretch. You can also use fusible woven stabilizer, but not non-woven, as it may come loose from the shirts in time. The pieces will feed better if you place the fusible side next to the feed dogs. The quilt should be machine quilted, because washing may tear or pull ties through because of the weight. Most of this info I found in a book called "You Did What With My T-Shirts?" by Pressing Matters Ink.
My heart goes out to you.