Parenting can be hard.
Parenting a toddler can be very hard.
I'm sure that parenting a toddler with additional needs can be so hard that it seems impossible at times.
I am deeply concerned about you H using dehumanising language as he shouts abuse at a 3 year old.
People use dehumanising language to exclude their victim from fundamental taboos about not harming other humans. It is extremely dangerous and could be a precursor to physical violence.
But he is already harming both your children and you with his current actions. Please get help from the police and woman's aid as well as whatever friends or family or support networks you can rally. You need to protect your children from this abusive adult who has made himself more unstable by having an unsupervised withdrawal of opiates.
Yes, parenting a young child with additional needs is hard. But he had to work out his feelings about that on his own or with a therapist, not attacking the vulnerable child who is finding it hard to navigate the world which must feel particularly threatening and scary this evening.
Lean on the courage you have been quietly developing over the past 3 years and call someone for help. If you don't know what to say, read out the posts you made here to the police (preferably) or your parents or best friend or women's aid/a local refuge/other helpine.
I know it takes courage to ask for help. But this is too big for you to manage yourself, the situation is too dangerous. He's unstable and aggressive and doesn't recognise your son as a person.