Make sure that the bike is the right size for the kid. By age 7, he could already be moving into a 24" wheel size bike.
How far of distances are you riding? What kind of bike do you ride? There are a number of "Road bikes" available for younger kids, either with 24" or 650c wheels.
One can stand up on the pedals from time to time to readjust body dynamics and rest the buttocks a bit.
Keep in mind that the angle of the seats are adjustable. I don't believe in making the seat level. For my road bikes, my general rule is to point the seat at the handlebars.
Anyway, try dropping the nose of the saddle by a few degrees. Too much, and one must also use the legs to dynamically hold oneself in place which may be a problem.
But, there may well be a happy medium place of adjustment.