If you are confident he will obtain a place at any one of three of the chosen non-selectives, put these on the CAF, and then put a selective as your first choice. Perhaps the easiest to get into selective. At the very least this buys you four months of time.
On the minus side 3a is, however, even at yr5, not-indicative of selective ability. But, on the plus side, 112 is not significantly below the pass mark. Further, he passed NVR, which is generally considered to be an indicator of mathematical potential. Also, if you trawl through various of the threads on this site you will see refs to children considered to be level 5's at the end of yr5 who scored less than 112.
Lack of school support is not good. But, by the time the appeal stage arrives, in four months time, the position may have changed. He may by then have demonstrated increased maturity, or competency, in maths. It even gives you time to work on the maths. Good luck.