There’s something called a light blocker strip you can Google that might help.

I got hunter-douglas Lightlock shades but a less expensive fix might be the install-yourself strips Chris has mentioned below. They too might lose their stickiness but they are not terribly expensive so you can replace them as needed or you can experiment with double-sided tape to re-stick when needed.

All that said, I dont think curtains are necessarily an expensive option, especially if your son really is losing sleep due to early morning light. Just buy a curtain rod with a ”french return” and buy black out curtain panels online. Your valance is so neutral in color that any color of curtain that matches one of the colors in the print will look fine.

https://www.wayfair.com/decor-pillows/pdp/ebern-designs-sauhal-adjustable-063-single-curtain-rod-w003199841.html

PS; If you can’t find a french return curtain rod that is shallow enough to fit inside the valance, then consider that the curtain panels on the side don’t need to be functional. Your son doesn’t need to be able to shut the curtains in the middle to block the light coming out the side. He only needs to be able to continue to raise the shade in the middle, and panels of blackout cloth on either side won’t interfere with that.

So at the end of the day all you really need is 2 long pieces of blackout curtain fixed under the valance. You can think about fixing them with velcro.

I’m sure a decorator would scream bloody murder but if I were desperate enough — and one can feel desperate to not be woken up at dawn every day by light in the room — I would just go ahead and put a french return curtain rod on the outside of the valance. How shocking. It’s a bedroom. It’s for sleeping. It’s not a guest room, and any guest in somebody’s bedroom who disapproves of the curtains is someone to be wary of having in the bedroom!

I can't think of an easy answer because of your specific setup.

Curtains are honestly the best idea here. Personally I'd rather take the valances down for this to look the best, but the french return rod that ital mover suggested might work. I don't even think they need to be blackout. Just get some thin curtains that you keep open all the time. When bunched open at the sides, they will be thick enough to cover the edges. As for the bottom, pull the shade down so it overlaps the windowsill better.

Plus, a tiny bit of gentle light will still get in (but not in a harsh way), which is a good thing. I caution you to not make that room total blackout. I did that before and it can really suck to not be able to see anything at all. Also, kind of weird but I grew dependent on a cave-dark room, so I slept badly whenever I was traveling.