I was with my child's father for 4 years. During this time he drank and took drugs. I began to do the same so I decided to end it. We broke up and he moved out. He was sharing responsibility for our 14mth old son but on a few occasions he was neglecting him due to his alcohol and drug abuse. He once took our son to his mothers place, proceeded to get drunk, took our son whilst still drunk on public transport, leaving him in the downstairs of my flat alone. Luckily I came down to see who rang my buzzer and found my baby there alone. When I contacted his mother and sister they knew he had been drinking and allowed him to take the baby anyway.
Stupidly, I continued to let father look after him for a while until my son hurt himself while in his fathers care and found him drunk on returning from work.
I stopped him seeing my son and also stopped his family having the baby on weekends. The grand parents do not agree with my decision and decided to contact my sons social worker without my consent. The social worker was charmed by them and taken in my their educated demeanor and suggested that I let my son continue to visit there even though dad will be there too and grandmother also drinks in the evening.
I want his father and his family to see my son but they have not proved responsible. I have advised them to pursue supervised contact but they haven't and I feel like the bad guy in this situation.
I also recently found out that his father cheated on me during our relationship, so it feels like I'm punishing him for this.

I can't see the situation clearly, am I doing the right thing? Should I let the family look after him unsupervised as they have promised to take better care of him even if his father is there or should I not take the risk and let them pursue supervised contact through a lawyer? Am I just bitter because he cheated on me , so am punishing him and his family? Please help!

Why is there a social worker involved and what is their duty to your son?

If the father is a threat, there should not even be question. The safety of the child is always first.

You follow the court orders in place, if it requires the father to have supervised visits make it, if it does not, go back to court and get it included

A social worker is involved because on one occasion I called the police when father was high on Ketamine (a drug) and I refused to let him in. My eldest son who is now 15yrs old witnessed his step-dads drinking, it affected our relationship and his emotional stablity and started getting in trouble with the law and running away. He now lives in a care home close by but I'm trying to rebuild our relationship. Therefore, my baby was also seen as a child in need, so assessments had to be done on me and father but father doesn't want to be involved with social services, so he was left out.

As yet, father has not sought contact through the courts, so there is no court order.

I recognise now that relationship has ruined my life and I regret having been involved with an alcoholic/drug user. He wasn't as bad at the beginning, I just thought he was funny, charming and drank on occasion, it was not until I was living with him and had his baby that I realised the extent of his problem. I drank with him (trying to keep up) and also did drugs with him (which I had never touched before him), but looked at the situation and told him to move out.

I used to think that I was a strong woman, well educated and a good mother and never foresaw myself getting involved in such a relationship. I put up with so much for love and a family, then he tells me he cheated too, what a joke, I've been a joke!

But all that aside the safety of my baby comes first.

Their duty to my son is to assess his well being in my care or whilst in his fathers care.

Like stated before, and you know, the safety of your child comes first and NEVER second guess yourself on that one. He is a baby and totally dependent on you for his wellbeing.
Not all woman use children as a weapon and it doesn't seem to me that you are or you would know. There probably is distrust for the whole family because of the father's behavior and the drinking.
Family is important and I do encourage you to work with the parents to find a solution to this. It is nice that they want to spend time with the baby.
They have to be responsible, they have to attend to the baby, and they CANNOT drink while they have him. You are the parent with all the responsibility and what you say goes, have confidence in that... trust that.
Work closely with your social worker, ask for her help. If you don't trust this one, ask for another one. If you have a worker with experience who is in good communication with everyone, it can be a real blessing.