I agree. Even at the lowest RFing weight convertibles he has 10 more pounds to rearface. There have been no documented cases of legs breaking due to RFing in an accent. There have been, unforunately, far too many documented cases of paralysis and internal decapitation caused by premature FFing.
My four year old, 38" 28# daughter rides RFing with me. Boy oh boy do her legs touch the seat. She can put them up on the back seat and rest her feet on the top (no headrest in the middle). She either crosses them, puts them up, or tosses them over the side of the seat. I've never heard a complaint that her legs are bothering her. In fact, a lot of kids are more comfortable RFing because their legs aren't dangling, which can cause numbness and tingling on decent length trips.
Definitely point your ped in the direction of the AAP's recommendation on this (
http://aappolicy.aappublications.org/cgi/content/full/pediatrics;109/3/550).
There are a couple of quotes about this I've seen, too.
"Bent legs don't break."
And
"Broken leg, cast it. Broken neck, casket."
On the link that Rebekah has for you there's an album of older/heavier children who are RFing. Definitely check it out. Here's Piper's page, from 14 months to 4 years, so you can see the progression over the time.
http://www.cpsafety.com/articles/RFAlbum/Piper.aspx
Wendy