I am so sorry about our grandson. My heart goes out to you and your family.
To make a tee shirt quilt, cut the front of the tee shirts very large and fuse a lightweight interfacing on the back. A teflon pressing sheet under the front of the tee shirt helps to fuse it. Then and only then, cut the tee shirts the size of the blocks you want them to be. Then you can add sashing or sew them together the way you want them. Do not skip the step of the interfacing. Tee shirts stretch and will not quilt well unless they are stabilized.