Pre-K is generally the year before Kindergarten, so when a child is 4yrs old.
At only 32lbs your child could still be REAR FACING, so he should in no way be in a booster seat. Keep him in a harness until he's at least 40lbs and at least 4-5yrs old. Graco Nautilus is $150 and can harness a child to 65lbs, generally to age 6/7 in height, and then becomes a high back booster and eventually a no back booster to 100lbs. He will never need another seat, and it will keep him harnessed for at least 3 more years. He's not a big boy at only 32lbs, so keep him harnessed. No 3yr old is readyf or a booster seat.
A forward facing seat is outgrown when one of htree things happens, the child goes over the weight limit, the ears are level with the seat back, or the shoulders go over the top harness slot. When ONE of these happens, the seat is outgrown and needs to be replaced ASAP.
Keep him harnessed, he's much too young and too light for a booster seat.
The law in Florida sucks, but remember laws are BARE MINIMUMS, and your child is worht MORE than the bare minimum. Keep him haressed!