The Centre for Education on Mathematics and Computing of the University of Waterloo offers free math problems of the week for grades 3 and up. You can sign up to have it sent to your email on a weekly basis. They also have booklets of past problems from 2018-19 and 2019-20.

CEHM Problem of the week

I may supplement the Grade 4 problems with Grade 5 ones to keep him on his toes 😉