Follow your mommy instinct because it is correct. By asking this question, this shows that you truly do not feel comfortable with your child riding in a booster seat. Your gut instinct is correct.
These parents thought that their 3 year old was old and big enough for a booster seat. She was in a crash and was killed. This was just about 6 months ago in Nevada: http://www.8newsnow.com/story/20596976/breaking-news-metro-3-year-old-not-in-proper-car-seat
A booster seat is designed to lift a child up for a better seat belt fit. It only sits on the vehicle seat and is not attached to the vehicle seat in any way. A child sits in the seat and is buckled. A child can only be protected in this type of seat if they sit straight up and back into the seat. A 3 year old child is extremely unlikely to sit still for the entirety of the ride.
In addition, the National Highway Traffic Safety Administration (NHTSA) says to keep 4-7 year olds in a car seat with a harness for as long as possible: http://www.safercar.gov/parents/RightSeat.htm
For a forward facing car seat to last your son a long time, look at the top slot height. This is because most kids will outgrow their car seats by height, not weight. The harness straps needs to be at or above his shoulders when forward facing. This means to get a car seat with at least 18 inch top harness strap slots to allow lots of growing room. It is dangerous to have the harness below a forward facing child's shoulders as it will compress his spine.
There are many options to keep your child in a car seat with a harness:
-Harmony Defender 360 (65 lbs with harness; 18 inch top slots)
-Evenflo Maestro (50 lbs with harness; 18 inch top slots)
-Evenflo SecureKid 300/ 400 model (65 lbs with harness; 18 inch top slots)
-Graco Nautilus (65 lbs with harness; 18 inch top slots)
-Graco Argos 70 (70 lbs with harness; 18 inch top slots)
-Britax Frontier 85 (85 lbs with harness; 20 inch top slots)
-Recaro Prosport (90 lbs with harness; 19" top slots) (This seat will not last as long as the Frontier as children will outgrow this seat by height).
All of these seats above start out as a car seat with a harness. Then you can remove the car seat and make it into a booster seat. The Evenflo Maestro's booster mode will be outgrown the same time as the harness mode is outgrown by height. This is because the headrest cannot be adjusted for shoulder belt fit.
My tip is to never worry about booster mode for any of these combinations seats. You can always buy a dedicated booster seat later and booster seats can be very inexpensive. The Graco Turbobooster (in highback) costs $50 at Target and you can buy that when your child outgrows their harnessed car seat (around age 5 to 6).
The Harmony Defender 360 can be found for $99 at Walmart, but it has a very short crotch strap which may bother your boy. It depends on the kid.
The Evenflo SecureKid's booster mode will give you about an inch of torso height room growth. In other words, after a child outgrows harness mode by height (when shoulders are even with the top most harness slots), then you will only get about an inch of growing room. This could be a growth spurt away or a year's worth of time.
I also recommend the Evenflo SureRide- it goes up to 65 lbs and has 20 inch top harness slots. This is a convertible car seat that goes rear facing to forward facing. In other words, it does not turn into a booster seat to be used later. It is about $99 and after your child outgrows it, you can just purchase a good dedicated high back booster like the Graco Turbo. The only major problem with the SureRide is that the harness is sooo short, so short that children will outgrow the seat by harness length! It should still last until a child is 45-55 lbs.
And don't forget to use the top tether on his forward facing car seat. Here is why it is important: http://www.youtube.com/watch?v=ULJ8Vx79Vv4
The video also organizes the different stages a child rides in. It made it a lot less confusing for me.
Also, ToysRUs will usually allow you to try the display model in your car before buying the seat. And surprisingly, I found that ToysRUs can actually be cheaper than Walmart/ Target or be marked the same price. Depending on your area, they may also have a different selection of car seats.
Have fun shopping!