It would depend on how high the bed is. Mine is high, I can just reach to sit on it ( I'm only 5'2" ) I can put a double (full) size quilt on it, about 80 x 93 - no extra for tucking under the pillow - and it is still 6" off the floor. I've been told that quilts that reach the floor are heavy to sleep under as the extra pulls down on one.
How old is your son, and what does he like, colours too. If he is small you might like to think of a very light batting. I have started using micro-fleece in mine, makes them very soft and cuddly. You will have to measure your bed.