My son turned 18 August 24, which means he was 17 almost the whole month so would he recieve his last survivors check in September?

Hi. No. Unless your son is still in high school or he's disabled, his child benefits will terminate effective with the month he reaches age 18. In other words, the last payment to which he'd then be entitled is the one for July that is paid in August (https://www.ssa.gov/OP_Home/handbook/handbook.04/handbook-0414.html).

Best, Jerry