Ica extension of stay is rejected online

Hi everyone, I am currently staying in Singapore holding S Pass. My boyfriend initially finished his course un Raffles college here in June 2018. So we went back to our country duringhis holiday and came back to Singapore on 5 July 2018. Before his school already cancel his student pass at ICA. So he entered with visit visa and our country is free visa. So upon entering, he showed his returned ticket which is 27 days from the entry date. Then the immigration officers brought him to the room and asked him alot of questions. After asking so many questions, they let him enter. So during his stay, we tried to extend his visa for another 30 days but unfortunately it was rejected without knowing any reasons. His plan is to get the job here. So can he still go to ICA office with the local sponsor for another try? Or he really has to go back to his own country and come back to singapore after a week later?
Please answer my questions.
Thank you.

His being interrogated on arrival was a STRONG warning sign, even though he was eventually admitted.
He should NOT attempt to enter again on a visitor visa within the next few months!
If he wants to visit Singapore again, he should better contact ICA or a Singapore embassy first and clarify the matter.

Thank you for your reply.
I just want to check one more thing.
So he bough the return ticket on 1/8/18 and already showed it at immigration before he enter. He entered to singapore on 5/718 so total numbers of day he stayed here is 27 until 1/8/18.
Is that possible to change retrun date to another two more days since i know that its not easy to come back after one week later?
Thanks again.

You sent me the same by PM (and I have already replied). Please refrain from doing that, as it only wastes my time!
I repeat in short: He can change his departure date as long as it is still within the validity of his visa.