My son hurts kids at school, can you help?

my son of five has not stopped hurting others at school, he is awaiting blood tests to see if he is hyperglosemic, as his father is, I am a single parent and have spoiled him rotten i love him so, but one day i promised him a present if he behaved good,and he did, this showed me that he is doing this on purpose, and has the ability to stop, my question is how do i become a more better parent, ive been to parenting cles, but was told i am doing the right stuff reward and praise etc, we share good times , he is the only child and i am an older mum, i am sure it must be me, i blame myself, today i was told he hit a boy in the face, and the boy is going home red and bruised, i want to give my son a slap on the backside, but am afriad he will tell school and social services get involved,i never hurt him. i am truley at a loss, the present thing was just to see if that would work and he could change if he wants to, please please any ideas ive done the step thing for four minutes, ive taken away toys. my son doesnt care. ive looked into emotional being his dad is not with him, but he visits once a month for a week,here at my house and i go out, im going wrong somewhere. someone tell me i dont mind brutal comments. anything just to make my son stop, he is clever and can be a good boy at school, but this is rare, thanks for reading.