What should i do?

Sounds like you’re already a single mother, what would be the difference if he wasn’t there?

I’m old school and I think that a child needs both parents. There are plenty of studies that show that children do much better in a home with a mother and father.
Can you have a chat with your partner about the future and voice your concerns? He’s a dad now… sure going out and enjoying himself is ok every once in a while, but he needs to put your child and you before everything else.
Drugs should be an absolute no. He needs to man up and start taking more responsibility.
If he has money for alcohol and drugs he has money for bills.

I was living like this. I left, but only after repeatedly asking for change and a break. Once it was clear he knew the issues we had but couldnt fix them I knew it was time to leave x

Have we had a conversation about this as a couple and now parents yet and if so how was that reacted to or received?

If not that would be my first steps.

Next would an ultimatum of change and proving it or be kicked him out if he’s just a BF and you pay all the bills, but have to be careful if he’s the type to take that as his out.

100% break up with him and kick him out.

What exactly are you going to regret? He's a piss taking bum and has zero respect for you. You and your son deserve so much better

It sounds like you’re doing everything and he does nothing for you. A relationship is a give and take type of thing and all he’s doing is taking. He sounds like a bum that’s munching off you. I know it’s hard to leave but in the long run I think you’ll be glad you did. If not for yourself do it for your child. Show him what a healthy relationship looks like and how a man should be treating his partner.

What exactly would you regret? He's been like this for over 2.5 years now, at least, he isn't going to change. The substance use is also a huge red flag. I hope he's at least not doing this anywhere near your child? It sounds like you're carrying almost, if not the entire mental and physical load. What is this person actually bringing to your life? Please ignore the comment about a child needing "a mother and a father". For one, that completely ignores perfectly healthy same-sex families and two, a child is MUCH better off with one safe, consistent and happy parent than one deadbeat and one overloaded and unhappy one.
I completely understand being unsure, it's a huge change and the prospect of being a single parent to a toddler is massively daunting. But this man sounds like he's bringing nothing but stress and negativity to both your life, and the life of your child. You deserve to be happy, can you honestly say you are now?