Should drug dealers be given more prison time? - By Mrs L

My son died of an heroin overdose in 2015. He had been to prison for offences connected with drug use and the only time I felt content was when he was in prison because I had always dreaded 'that phone call' telling me my son had been found deceased.
That day did eventually happen.
When I go to the shops sometimes of an evening; I see the dealer stood lurking serving addicts. He was the same dealer that sold my son heroin. He is a well-known dealer and the Police have either given up chasing drug dealers or they have caught him but the courts continue to save his day and give him a 'get out of jail free card'. My son was 19 when he died.
My opinion may cause some uproar but I will continue to give it.
I would like to know how the prison's rehabilitate drug dealers? When someone deals drugs; they make money; far more than they would working legally in a warehouse or on a production line.
How do the professional bodies rehabilitate them? The prison sentences handed to drug dealers aren't so hefty; and I know that our lovely community dealer has in the past been to prison numerous times only to come out of jail and start his little empire where he finished off.
I would also like to know why drug dealers aren't charged with manslaughter when they sell heroin cut with other deadly drugs such as Fentanyl? I would like to hear opinions from people who agree or disagree with me. Should drug dealers be served with bigger prison sentences?

Kind Regards

Mrs L