I'd go for Jammers. I have a pair that I bought for my boys Speedo Endurance ones - they are age 14 have a drawstring but still have the tags on as neither my 12 or 11 year olds can squeeze into them (they are both solid boys) - the waist is about the size of one of their thighs - I reckon those style/make would be ideal. Plain black except for the logo.
Is it maybe the getting changed in front of everyone? My 12 year was getting quite excited about having some downstairs hair but I don't think he is typical in shouting about it to all and sundry!
I have a very skinny ds 13 too and found some in sports direct. They had some close fitting longer ones and shorts style and definitely needs drawstring waist. Not speedo briefs like pants, more like a loser fitting boxer short, or ones that come down thigh.
Having trouble finding the right trunks. They have to be figure hugging ones, apparently. He refuses to wear the ones I've found. He wants them long enough - coming half way down towards knee length. But these don't have drawstrings to tie up and fall down when he dives in (!). The trunks with drawstrings are too short - briefs really. I'm at a loss. Usually I'd say just wear one of the many pairs we've tried - like it or lump it, but last week he bunked off swimming and lied to us (very out of character, and was really upset when we worked it out by the lack of chlorine smelling, and damp stuff in his bag). He said it was just because of the trunks. I'm not sure that is the full picture but want to take this out of the equation by getting trunks he's happy wearing. Any ideas? Thanks.