since ironing the decals on the shirts can create a huge mess- i would (slow-down)
he should first stablize the squares he has cut out- with a lightweight stablizer applied to the back of each one- measure the largest square- decide if it's the size he wants- or if he wants it a bit larger- add sashing strips to make it the size wanted- then add sashings to each smaller square to bring them up to the size of the first square- (using the background fabric) fill in with additional squares/rectangles of background fabric where needed to create the layout wanted- bring it to the size wanted-
sew together in rows- then into the top ... the fleece backing is great- adding the backing can be (pillowcase style) or by regular quilt sandwich-- then the quilt can be tied- or stitch in the ditch-ed around the squares- great beginner project!