I really like what you've done with this quilt. It looks very comforting as well as very reminiscent of a young man. It is quite a challenge using all of those different fabrics.
If you ever sew with knit fabrics again, you would be better off using a very narrow zigzag stitch. This is because knit fabrics stretch and a narrow zigzag allows the stitching to stretch a little too. A zigzag would have given your seams more flexibility. However, because the gray is interlock, it's probably not as serious as with other knit fabrics. This is because interlock is soft and can flex a lot around the stitching line. With many stretchy knits, if you sew a straight line and then put stress on the seam (for example, by pulling both ends of the stitching line), the thread will break because the fabric will stretch while the seam does not.
I still think you will be okay without additional quilting. The good thing about interlock is that, if the seam does open up, it's pretty simple to hand sew it back together by hand. Because it doesn't fray, you will still have a seam allowance to tuck in.
If you have scraps left of the interlock fabric, you could create a test strip to kind of test the seam. I would seam two strips of the interlock, sandwich it with some leftover batting and backing, and sew quilting lines 2" apart just as in your original. Then twist and pull on the strip every which way to see if the seam holds up against stress.
If the thread in the seam snaps, you might want to consider adding some quilting. I wouldn't add another line in the same direction, but maybe add some perpendicular lines (running east-west to your current north-south) to cross that long seam. They could even zig and zag around blocks, staying in the gray interlock. This would strengthen the entire quilt and still be in keeping with the masculine nature of the quilt. In the photo above, I could see a quilting line coming from the right, following in the seam to the middle of the next gray strip, then proceeding on to the left. You wouldn't have to do this in every block; you could skip some. Or maybe not zig zag, but keep moving up when you encounter a block, making them long zig zag diagonals. Then another line similar, starting a couple of blocks down. This would be a modern type of quilting design that would not detract from what you already have.