If he gets too hot, I would use regular cotton fabric for the back as flannel is very warm. Quilts without batting were referred to as Summer Quilts as they were lighter weight. Certainly you can make them without batting, but you need to carefully press yur seams so they don't make "bumps" that show through the backing fabric.

I made a flannel rag quilt without batting. It was plenty heavy and still very warm. My son-in-law is the same. I suggested we buy a lightweight bedspread and make runners and pillows as accents

Quilt without batting
 

I use a quilt that was made by my aunt. It's batting is a sheet. So there are 3 layers of cotton. Light weight and comfortable all year.

I just made a top and put a piece of fabric a little bit heavyer than regular cotton and love it for our Texas Summer. No batting

Don't know how much you can tell from this picture but, I made my dad a wheel chair quilt with just the front (appliqued and pieced) and put flannel on the back. Not too heavy but still a good weight. I ended up tying it .

My daughters make quilts all the time for their kids with no batt. I admit that's difficult for me to do, but their's always turn out nice. And as I think about this, my favorite snuggle throw has no batt.

I have made several quilts /flannel as the batting. Just be sure to wash the flannel before you use. Layer and quilt as usual. My grandmother had several & we called them sleeping porch quilts or summer quilts. Very nice for summertime use. Good luck.

I can't see why you couldn't make it without the batting, but would avoid flannel because it will make it too warm. I received a mug rug from a new quilter in a swap and it is made without batting/quilting. It is very pretty so I am using it as a wall hanging instead of as a mug rug. Coverlets are perfect for summer and/or warmer climates.

A couple of years ago I made a top and just wanted a Summer Quilt--without any batting. The girl who did the machine quilting said it was okay when she looked at it.
When she got done she said it would have been better to have something for a batting; that it would have been easier to quilt;even a sheet.
I love the quilt, and it looks nice; just the quilting was the issue.
Enjoy!
Mariah

I live in Southern Arizona where it's warm most of the time. I'm just finishing a lap throw for my sister who is undergoing chemotherapy and gets cold easily. I used no batting. Just quilted the top to fleece. It makes a very nice, light quilt. You should be fine with flannel.