This may be more "girly cute but still boy" than you want to go, but I personally find I *feel* cute wearing anything pastel or bright.
You can keep your outfit in the whites, soft yellows, and baby blues, and it still won't feel even remotely masculine or grown up.
Get a couple of well fitting bright colored shirts with cartoon characters. Don't get a dark blue or brown or black shirt. Get ones with bright or pastel backgrounds. You may have to shop the women's section, but there are many that are cut just fine for men and when worn look more childish than girly. One of my favorite toon shirts is a Tinkerbell shirt I bought in Disneyland!
Oh a cute backpack can help too! I have a Steven Universe Hamburger backpack that *always* gets commented on!
Shorts are good for feeling little, especially bright shorts (white, pastel, bright colors - never khaki, brown, navy or black) especially with long socks, especially with over knee socks, especially if they're pastel stripes!!! ^///^
Get cute shoes... if strap on are too girly, then even white or pastel blue keds are hella little cute.
You can also lean into the "old fashioned boy" cute. I have a couple pairs of bloomer / pumpkin pants with bows on the ends, and a couple frilly "little lord Fauntleroy" blouses... though I don't wear these outfits out and about! ^///^;;;
Here's a couple of toon-ified pictures of outfits I actually do own and wear (uh... I mean I'm not a actually a bunny, but the rest of the pic is accurate!)
https://www.furaffinity.net/view/22506025/
https://www.furaffinity.net/view/23256993/