Lightweight quilt batting.
 

I'm making a quilt for my 3 1/2 year old grandson and he is always hot. What can I use other than traditional batting to make the quilt lightweight.

I have heard that wool batting breathes better in the summer, as well as being cozy for the winter.
You could also use flannel for the batting, or just use a flannel back, and leave out the batting completely.

some were talking about Hobbs Thermore (Thinsulate). very thin poly batting. love it.

Yep, I'd go with wool.

I go with the flannel with no batting or just a cotton backing (summer coverlet)

Quilter's Dream Request. It makes quilts so soft. Also do not over quilt to keep a quilt more "foldable".

I keep hearing about wool.

You said "other than traditional batting" -- so I won't suggest types of batting. Honestly, why not just make him a coverlet rather than a true "quilt"? A coverlet is 2 layers with quilting (I've always seen them with 2 layers of quilter's cotton -- or sometimes one layer of linen & one layer of cotton if it's for an adult bed. Sticking with cotton front & back will keep it cooler than backing with flannel). They are very popular in warm climates like Costa Rica & are every bit as beautiful as a "quilt". Unless you are planning to enter it into a competition, there's no rule that says you have to have 3 layers in order to quilt it.

Also, keep in mind that the more densely you quilt it, the heavier (and hotter) it will be. With a 3 1/2 year old, you need to quilt it closely enough together that it will hold up in the wash (generally, that seems to be no more than 2" spacing) ... but I probably would try to choose a design that is at least 1.5" apart so that it stays soft, drape-able & cool.

Just use a soft fleece ( minkie) for the back...no batting needed.

I can relate -I made a Star Wars quilt for my grandson, and he, too, is always hot. I used Soft & Bright by The Warm Company -it is a needled polyester batting, very compact and low-loft. I just love it, and I use it almost exclusively for all my quilts. It can be a bit pricey, but I've always been able to buy it on sale at 40 - 60% off, so the price turns out to be much better than a lesser quality batting.