Fusion is suppose to be the best plastic paint around. Ive heard it many times over. Ive never used the stuff myself but have seen pictures of what someone claimed was Fusion and it looked sweet. As for strength I heard if painted correctly its great.
So maybe your son did not paint it correctly, also I belive they recommend plastic primer. Theres also many steps involved in painting plastics. sanding, primer, sanding, primer, painting, sanding, painting, sanding, painting, sealer 2-3 times.