One of the ladies in my sewing group this year started making Bears from shirts. There are free and low cost "Memory Bear" patterns. She adjusted her pattern and includes a pocket on the front in order to hold a keepsake if desired (pockets often have company names or logos etc). Her design uses one large shirt each, but she has also done them using multiple shirts to make multiple bears, that is one arm is from one shirt, the body from another, and so forth so that all the kids had pieces of all the shirts.
I have seen many quilts made from basic office shirts, mostly basic white with blue and other solid color shirts, plus the subtle striped ones, along with some plaids and maybe some Hawaiian or casual Friday options (which for my engineers usually meant no ties and short sleeves instead of long) Basic repeated designs using simple shapes seem to work best in my opinion, so traditional patterns like stars and 4- or 9-patch units work well.
There is a lot of fabric in shirts, the backs are usually in good shape even if there are stains on front or worn areas. Just the back is at least a fat quarter, adding in the rest you have about yard+ per shirt. To practice or if you need a few more, you can get good some good deals at thrift stores for nice usable fabric for around $2 a shirt (and less if they have bargain days or "buy the pound/bag" options. Check out the men's X+ sizes! Depending on what your dad's shirts are made from, I'd consider any blends to be suitable to use with the full cotton, but you might want to look at the labels when buying extra shirts.