Well you definitely came to the right place!
Your fears about the legs are a common fear people have, but the legs are totally fine rear facing. Leg injuries just plain don't happen when rear facing (even bigger kids) but are actually common with forward facing children. But even if a leg did get broken, it's fixable and a broken neck from forward facing is not. I heard a great saying on here: "broken leg, cast it. Broken neck, casket." Really paints the picture!
At 7 months your child's feet may just be starting to touch the back of the seat. As his legs continue to grow he'll bend his knees and find a place that's comfy for his feet--some cross their legs, hang them over the sides, or walk them up the back of the seat. It's actually really comfortable, like sitting in a recliner. When kids turn forward facing they often start complaining that there's no place to put their feet because their legs are just dangling there!
What you need to get next is a convertible seat. It will keep your son rear facing for a couple more years, then when he outgrows it by height or weight rear facing, you just turn it forward facing and continue to use it a couple more years. You want one with a 35 pound rear facing weight limit and a tall shell (the "back" of the carseat) so that the child won't outgrow it too soon by height. A 50 or 65 pound forward facing weight limit will help make sure your child doesn't outgrow it FFing before he's ready for a booster.
A few commonly recommended seats here are (in order least expensive to most): Cosco Scenera, Evenflo Triumph Advance, The First Years TrueFit, Sunshine Kids Radian, Britax Marathon/Decathalon/Boulevard.
There are many other great seats out there too, but stay away from anything with a 40 lb limit as it will be outgrown too quickly for most kids. Also stay away from the shorter shells because it will be outgrown too fast by height. The best seat is one that fits your child and your car and is used properly every ride!