Boy clothes are HARD to find! If you sew, you can make them fairly quickly. I've made a few pair of pants. Some AG and OG shirts for 18" dolls can be used for boys too. While out shopping, keep in mind colors and patterns that will work for boy dolls : blue, black, gray, brown, red, white and green. Of course denim jeans and denim shorts work too!
Here is what I have in my doll closet for boys. I have used Molly's white camp shirt and Kit's button down shirt.
The white t-shirt is Hudson's meet shirt, the brown shirt is a stretch JG t-shirt.
The leather jacket is JG and the blue hoodie is from Hobby Lobby.
These are DIY pants.
Hobby Lobby brown denim cap, AG beanie, JG sage cap, Addy's boots and Hobby Lobby white sneakers.
Don't forget accessories: hats, backpacks, skateboards, sports equipment and guitars.
You can make historical clothes too. Simplicity has patterns for Bitty Twins.