1) Visit the doctor again with your child. Have him thoroughly diagnosed. Ask relevant questions i.e. how come without internal bleeding, the child complained severe pain several hours later? what could be the worst etc.. you name it. The best is to prepare yourself with a question list. Work together with the doctor aimed at obtaining a professional report.
If this report indicate that your boys complain (pain) does not necessarily have anything to do with the "kick", stop here but not sending your boy to that school;
2) If the medical report indicate or suggest that your boy's complain is likely associated with physical contact -- kicking, bring this report to see the police again, have your document (simplified english version of your post) submitted to them and ask them to step in to any degree they could. Better with your lawyer to visit the police.
3) the next time to see the school administrative staff, the best is to have your lawyer, and the police officer, also the medical report. At least to have lawyer's letter with you.
This will take tremendous committment, and still no promise of the justice.
I have seen such matters a few times. at 7 yo it's quite possible for a child to estimate the possible consequence/punishment and to make choice to manipulate the fact in order to avoid the punishment. Getting a witness promptly may not help either. In my previous observations, the witness -- usually those young children of same age, are very easy to be manipulated by the school staff or their parents. You know why and how.
Teach a child to be firm and loud to stop the attack at the first moment works the best, but may not be easy for all. Still worth trying.
