I'm not sure he means heavy as in weight or just warmer. I think I'd use a poly batting and minky on the back. That would make it much thicker and warmer.

I've used Warm & Natural with a lot of success (over 100 quilts) It is denser than poly & therefore heavier. You could also put flannel as your backing fabric & yes, you can use flannel back on cotton front :) I especially like to do that with baby quilts & quilts that are used for a winter throw. The flannel makes it warm & snuggly.
*W&N is great for machine quilting, not so fun by hand. If you're quilting by hand, try 100% wool - it's a DREAM to quilt - like putting your needle thro butter! Wool is warmer in winter & cooler in summer.
Hope this helps -

Back it with polar fleece and don't use any batting. I have friend who makes lots of super-warm quilts this way.

So here's the question that needs to be answered. Does he want a warmer quilt or a heavier quilt?

My DH went as far as to buy a weighted blanket - 15 pounds for a 145 pound man. He loves this, but he does have some medical issues that the heavy blanket addresses. I wouldn't last a minute under it.

If your grandson wants a warmer quilt, use a lofty polyester batting.

If he wants a heavy quilt, use a cotton batting (Warm and Natural, or my fave, Quilter's Dream), as lofty as you can get. Then use a heavy flannel for the back. Even a lightweight denim might work if he really wants it heavy.

Personally, I like a heavy, but not necessarily warm quilt. I like the weight, but I also get hot. I use Quilter's Dream, in their mid-loft, which is enough weight for me.

Thank you all,have something to think over. I do know he wants the school logo on top and blue from it on the back both will be cotton. Wish me luck I am finishing up one now for my granddaughter so his is next.

I made an I Spy rag quilt with cut up denim jeans as the backing. That thing is as heavy as an xray blanket and wears like iron. The grands drag it around everywhere.

Just another consideration.