the transition in toilet training can be difficult in some children with a lot of individual variations , some children find it easier to withhold urine or stool than to go bathroom, some children enjoys playing and forgets to ask to go to bathroom or avoid that and they squat and hold their legs in a bizarre way to withhold urine .
it's very common in this age group , but you can try few things :
1- increase fiber diet , he already drink a lot of fluids , so it's a good thing I'd he is gaining weight .
2-make him go to bathroom after each meal , to make use of the physiologic gastrocolic reflex , in which when food enters the gastrointestinal tract , the
colon make peristaltic movement and then passes stool.
3- if you see him do squatting take him to Bathroom .
4- if does go to playgroups and pre-KG advice his supervisor to take him to bathroom when she squatting or after meals .
I hope this helps