B.W.
The BARE MINIMUM is 4yrs AND 40lbs. He's not even close. :)
A forward facing car seat is outgrown when one of three things happens; the shoulders go over the top usable harness slot, the tips of the ears become level with the seat back, or the child goes over the weight limit. IF even one of these things happens the child has outgrown the seat nad it must be replaced.
Graco Nautilus is $150 and will harness ot 65lbs and then booster to 100. Most kids can use the harness to age 7.
He's much too young mama, don't make the switch. Keep him harnessed for at least antoher couple years.