Here's a boxer briefs for you little boys. Use t-shirts, mix and match patterned stretch fabrics or just go all one colour. This pattern is for boy's wearing size 2.
These boxer briefs are made from stretch knit fabric, make sure the fabric always stretched width ways, around the body. If it's not stretching that way there's no way your little dude is going to get his junk and butt into it!
You should also check this Men's Boxer Shorts FREE Sewing Pattern and Tutorial.