E.,
As a music teacher of elementary-aged students, I first want to say, good for you for getting your son into the arts! Since he is so young, he is getting an early start on a wonderful experience that most kids will not get.
As for letting him skip practice and just play with his teacher, that is not only a waste of your money, but a waste of his teacher's time. If he enjoyed playing in the summer, think about when he played and for how long? Did he play only in down time before you left to go somewhere? Did he play a little bit before dinner time? Try to re-create a small window of time like that where he will see it is time to play his violin and where it will fit into the routine more easily. Also, since he is so young, I wouldn't recommend having him play for more than 10 or 15 minutes at a time. This way he will be able to practice what his teacher gives him, have some fun, and then be finished, without feeling overwhelmed. Even practicing just every other day would be better than not at all, if you plan to keep up with the lessons.
Otherwise, maybe wait a few more years until he can start playing the violin in his school strings program (usually starts in 4th or 5th grade).
Good luck!
B.