At 35 pounds and 40 inches he sounds kind of young. You'd want a higher weight harnessed seat, not a booster.
Every seat out there is as safe as another provided it's used properly. So you won't find seats ranked by safety. They all pass the same testing.
However, the seats with higher slots and an over 40 pound weight limit are what you want. That leaves you with:
Britax Marathon/Decathalon/Boulevard: FFs to 65 pounds/17" top slots.
Britax Regent: FFs to 80 pounds/21" top slots
Sunshine Kids Radian: FFs to 65/80 pounds (different versions), 18" top slots
Recaro Como/Signo: FFs to 70 pounds, 19" top slots
Evenflo Triumph Advance: FFs to 50 pounds, 17" top slots
Apex 65: FFs to 65 pounds, 17" top slots, booster to 100 pounds
Those are it, and they range from $100 to $300.
How old is your son, what seat is he in now (the 40 inch limit is a guideline, as you can tell it's torso height that is important, not overall height, your son may have outgrown his seat, or may have room left), and where are his shoulders in relation to the top slot? That may help us determine if he's a light child relative to age, if he has a long or short torso, etc. Also, what is your budget, and what car do you drive? Do you have any other children younger than your son who would use a seat after your son has outgrown it?
Wendy