Classic Briefs and Boxer Briefs (affiliate link) by Peekaboo Patterns and made the briefs with a faux fly. I made them with a size 2 width and size 3/4 length. I skipped the side detail and sewed French seams instead. I reversed the French seams so the the seam allowance ended up on the outside so that there would be less fabric to irritate on the inside. The only seams on the inside are from the bands which I'm hoping won't bother Henry at all.
I was really worried that they would fit and look good on Henry. I have sewn a lot with knit fabric and have come to distrust knit patterns. I haven't used a lot of indie knit patterns, but of those only a couple of them are good. I can tell when an indie designer knows how to work with knit fabric or not based on the pattern. Anyway, these fit Henry perfectly! The leg bands and waistband were snug without being too snug but pulled in the briefs like a knit band should.