concentration issues for my 5 year old

I would be very gratefful for any advice regarding my 5 year old son. He has a persistant cough that sounds quite chesty although no expectorant with it, and it settles at night. His concentration and focus is terrible. I wouldn't say it so much as behaviour issues more the lack of listening and HATES being told No. He lacks confidence and yet is very bosterous. At school if he is not first line he hits out at the other children but not maliciosuly just a case of 'your in my space, move' it's very black and white to him. He is an only child and i'm a sningle mum so i struggle becuase it's tiring constantly having to cajoole hime. I appreciate my parenting skills probably leave a lot to be desored and i'm just finishing a full time degree so time has been non exsistent however i am working on my parenting and time will be beter when i go part time. He has become very afraid of the dark and has trouble playing with other children but doesn't liek being on his own, however i am shying away from social groups because i'm worried how he will be. He seems to have gone backwards like having a 2 year old showing off and having temper tantrums. YET he is the most adorable little boy too, very kind ,very loving and very thoughtful, but i have to constantly remind him to focus on one thing even putting his coat on he can get distracted and his coat stays on half on.!!! At school he is struggling to remember what some letters are and struggles in a class of 30 but then again so would i and he is only 5 but i don't want my little boy to struggle but i feel if i don't sort this out now i will have major problems in the future. I did see a homeopathists and see prescribed Tub bov, 200C once a night but his behaviour got worse and when he coughed he said it tasted like blood but there wasn't any blood so she switched to taran 200c once a night but again it didn't make any difference. the homeopathist said he was a typical tuberculous child he does have a line of hair down his spine which apparently is a trait.
Please help.
many thanks
Flori