hii all, I need help,i dont want to be seen as a bad mother

well,my boy was in P2 last year and he was in Band 1 for his subjects. But now since he went to P3, his teacher has started to complain that he is not concentrating, he gets many silly mistakes for English & Maths ,and there is many corrections for him. When he comes from home at 2pm, he prefers to play computer, i phone, watch tv, play lego...i tell him to rest till 4pm ,and later I will do revision with him, but he cant be bothered. Also he has become defiant and talks back to me.
I have tried talking to him to ask if there is any problem,and also hit him on his butt at times, but nothing works,.If I ignore him,he keeps saying " mama ..mama .." and i tell him not to talk to me, he starts to cry. I cant see my boy cry,it hurts me.
His SA is in May ,and I dont want his grades to slide.. please help me... my husband doesnt bother about my kids education. No point talking to him.
I have been experiencing that every year has changed their behaviours so fast (feb).In P3, my son also started talk back learning from school.I also worried.Talked to him.Now in P4,he told to his friend don't use bad words.His friend told him that he never say any bad word.My son told me that I have to get aggrement every time otherwise people deny easily.
I think close relationship will help in discipline.When he cried u just think I am doing good for him.Don't regret,it will spoil him.be strong.U can let him to play computer in the weekends.If I allow him to play computer he just finish and go to computer.no concentration in studies.Last few weeks I implemented no computer in weekdays.