I struggled for 3.5yrs with this exact problem with Ds1. He absolutely refused to cut his hair and it was very curly. In the end I got so sick of trying to get the dreds out because he put food or sticky stuff or toy parts in his hair that I got it cut a little. Then he wanted me to cut it all off because he hates washing his hair.
I found keeping a good eye on if there is anything sticky in there was a help; as was washing it every night and running a comb through it before rinsing.
My Ds3 has got even curlier hair than Ds1 and it already gets knots and its not even that long.

Atleast with boys you can cut their hair real short if it gets too much lol

(Oh, Im the one they got curly hair from. I think curly hair is a PITA unless its really long.)