My son turned five the end of November.He's about 37-38 pounds.He still fits well on his racecar bed, and likes it very much.
Check the toddler bed weight limit.. that is a good indicator... another is size of the child. If he cannot fit in the bed without his head hitting the top or his feet the bottom.... then he probably has to have a twin bed. However... at at 5 he's old enough and most 5 year olds could handle a twin bed without the bed railing... so anytime between now and when he literally out grows the bed is fine.
You didn't have to put him in a toddler bed. I'd imagine that when he starts school and is then a big boy he will want a twin(you can still use racing sheets). As long as he fits and likes it you are good.
I would let him continue to use the toddler bed since he likes it so much. I think that you can get racecar beds that are twin size though.
