Looks like this is more of a problem with his parents than "表弟" himself. It is lots of work to educate a problem kid, but achievable with supporting parents. It is, however, almost impossible to educate problem parents.
With your 舅父舅母 not caring for their own child, I don't think you can do anything that can help.
I feel so sorry for your mom. If I were you, I would send "表弟" back to his mom in China.
in case,可能唔係得你舅父一個係咁,其實好多家長每當面對呢D問題嘅時候,佢地第一個反應多數都會講:我唔覺得有問題喎!個個細路都係咁嫁啦 阿邊個...阿邊個...個仔重衰個我個仔啦!有咩出奇啫,最後一施再施!其實好多時就係因為D父母唔肯認真正視呢d問題,勇敢面對,本來可以早早治療,最後損失果個唔係話你個仔有問題果個,而係你個寶貝仔!
你表弟行為好恐怖, 但我唔同意將佢送番大陸或話之佢唔理, 因為佢 ge 行為再下去, 大個咗分分鍾會變到 “大奸大惡”, 對社會構成威脅. 佢其實好可憐, 成長在一個冇家庭温曖 ge 家庭, 甚至可以話係有問題家庭, 可能再加上本身有 d 問題 e.g. 語言障礙等. 佢仲細, 應該有得救, 但必須尋求專業輔導, 佢 ge 情況應該唔係一般人可以 handle. 有個 bk mama 話送佢去條村到寄宿, 亦係好提意, 你可揾吓有關資料, 幫吓個小朋友.