give them to a charity shop.
find some sturdy sandals, with more room for growth, let him choose the ones he likes., or at least feel that he has chosen them. and don't spend so much. many parents go to clarks for feet to be measured then find that there is nothing suitable in stock that day, then go down the road to shoezone, and get something reasonable for a few months' wear.
www.shoezone.com/Products/Red-Fish-Boys-Blue-Easy-Fasten-Sandal-29608
i'm thinking something like this maybe. but this one may be too big. good luck. never force a child to wear uncomfortable shoes, their feet and toes are so delicate, can easily get malformed, as i know with my bent toe.
who is more likely to know whether they shoes are comfortable or not, the person wearing them, or parents because they paid a lot, or shop because they measured him.