My 9-year-old son weighs more than 90 pounds, but the adult small lifejackets are too big for him. Anyone recommend something for these in-between sizes?

Try Barts watersports or Overtons they have kids lifejackets. I just pick one up for our son who is 9 also and weighs close to 90 pounds also and bought the jacket that's for 90 to 120. It's the neopron or something like that and cost basically $55 before taxes.

Sterns makes a Very/small size might even have a few if you want them.

Here ya go, Oveton's has "teen" jackets specifically for 90-120 lbs.

http://search.overtons.com/?Ns=p_image_sort|1||p_purchased|1&cname=Teen-Life-Vests-90-120-lbs&Ne=1000&N=2553