Flannel is a looser weave cotton fabric than the regular fabric we work with. For that reason, prewashing is highly recommended to make the weave a bit tighter, and not have so much shrinkage after you have washed the quilt the first time (or the recipient).
THe nice thing about sewing with flannel, if it isn't pre-washed is: It is going to shrink, giving it a very antique, well-loved look and will actually become more "substantial" feeling after it has been washed.
Flannel will probably shrink more per inch than regular one hundred percent cotton, because of that loose weave, or it may be more accurate to say: fewer threads per inch in the flannel. Just keep that in mind as you work.
Your quilt for your son was very good, and he is a handsome boy.
You have a lot to be proud of.