1995 Chevrolet Chevy Van

SEAT BELT ON REAR PASSENGER-SIDE SEAT CAME UNLOCKED DURING TRAVEL (THIRD OCCURENCE IDENTIFIED). THIS SEAT IS USED BY MY 3-YEAR-OLD SON WITH A CAR SEAT. TODAY THE CAR SEAT AND MY SON WERE THROWN FORWARD OUT OF THE SEAT WHEN I STOPPED THE VEHICLE AT AN INTERSECTION THAT IS ON A DECLINE GRADE. MY SON DID NOT APPEAR TO BE INJURED AS HE WAS STILL SECURED IN THE CAR SEAT, HE WAS, HOWEVER, VERY SHAKEN UP AND SAID THAT HE BUMPED HIS HEAD. TWO PREVIOUS OCCURENCES OF SEAT BELT COMING UNDONE DURING TRAVEL HAVE BEEN NOTED BUT WITHOUT INCIDENT. SPECIAL CARE HAS BEEN TAKEN TO ENSURE THAT THE SEAT BELT IS SECURELY LOCKED WHENEVER THE CAR SEAT IS TAKEN OUT AND PUT BACK IN OR ADJUSTED IN ANY WAY. AS THIS IS GENERALLY CONSIDERED TO BE THE SAFEST PLACE FOR A TODDLER'S CAR SEAT IN A VEHICLE SUCH AS THIS, I AM HESITANT TO MOVE THE CAR SEAT TO THE OTHER SIDE OF THE VAN (BEHIND DRIVER'S SEAT). ALSO, I DO NOT KNOW IF THE IS A PROBLEM THAT COULD ALSO OCCUR WITH THE OTHER SEAT BELT AS WELL, AS WE DO NOT NORMALLY USE IT. BUT, AS MY SON HAS A SEIZURE DISORDER DUE TO A TRAUMATIC BRAIN INJURY, HIS HEAD MUST BE PROTECTED FROM IMPACTS. UNTIL I CAN DETERMINE THAT THE SEAT BELT IN QUESTION CAN BE USED SAFELY I WILL HAVE TO PUT MY SON AND HIS CAR SEAT IN THE OTHER SEAT. WHAT CAN BE DONE TO CORRECT THIS PROBLEM? HAS IT BEEN NOTED BEFORE? COULD IT BE RELATED TO THE CONVERSION PACKAGE ON THE VEHICLE (THROUGH TRAVELTIME VANS)? WHERE DO WE START TO GET THIS DANGEROUS SITUATION CORRECTED? *AK