You wrote about work, however, I presume there will be other things -- like cooking -- that you would need him to increase his share of doing.
I have been in what seems a similar situation in what seems approximately the state of your partner.
My suggestion is that you two take time for an extended conversation about what is happening, what will happen and what is needed to do by either of you.
I assume it is the first baby for both of you, so both of you -- but him especially, might be in for a shock, since it is will 99,9% change your mode of living drastically.
You sound as if you are realizing much more what it will mean than he is (which is mostly the case historically, I'd say).
There are two basic things to make as clear as possible:
1) The way of living you two had, will be gone with the baby. For both of you. Gone for good, or at least until approximately when the kid starts going to the school (yes, preschoolers who go to kindergarten still frequently don't let their parents have their sleep at night).
If he is to be your partner and is thinking seriously about being father, rather than a college roommate style boyfriend, he will have to take up certain extra responsibilities, some of which will start even before the child is born.
One of the responsibilities is earning enough money for you THREE to survive the time while you will not be working and after that for some time probably working less than before. Not only you will be busy and tired with the baby, but there will be an extra person in the house, who will have his needs, which will cost money to fulfill, too.
I would not concentrate on him spending time in Youtube, rather than just on whether he can raise his earning power.
If he can do it while still goofing around, more kudos to him.
You together with him would write down how much money each of you is earning, how much you are spending, how much extra will be needed when the baby arrives (nappies alone can add up to substantial amounts per month) and the amount your household income goes down when you can't work.
Which will together show how much more he needs to earn in order to support himself and his family -- namely you and the baby.
2) However, as I said, merely earning more is just one step. I expect that you might reasonably expect him also to pick up house workload.
Therefore, if he is not only working less, but also doing only a limited amount of household tasks, I would advise you to think of the total sum of things that you think need to be done regularly -- you listed cooking, but there are others and discuss with him, whether he is ready to take on those, which you need him to.
In my mind, it should go sort of all together with earning discussion, because these are all extra responsibilities that he will need to adapt to, and he might feel resentment, if there keep cropping up more and more things "nobody told him about", but which he suddenly needs to start doing.
Realize that some of the things you might have been doing, and which he doesn't do or doesn't do without reminding, might seem quite unnecessary to him. Many men, myself included, are partly oblivious to what we eat, whether the floor is spotless and if the bathroom sink is white or not really. There are exceptions, but by and large in my experience we really don't care much.
Therefore, there might be things that you think need to be done regularly, but he doesn't. You'll need to somehow come to agreement on this. Again, in my experience, it has worked best, if we can meet somewhere halfway and agree. If that agreement is there, it is much easier to divide tasks, because then part of those taks won't feel to him like to be done simply to oblige you.
This might come as big shock, but if he is going to remain your partner, he needs to "put his easy days down" same as you are doing both with your job and baby, and to understand that the era of pattering around slowly and half-awake has ended, just like teenage years and college does. Welcome to the wonderful world of parenthood.
Talking with him and helping him realize all the things he has to start doing (and will have to keep on doing for years) is the primary thing, but concentrate more on the goals to be achieved rather than behaviour to be stopped (ie. "need to earn more" rather than "stop watching youtube").
PS. Also, I would form those things more as questions in terms of household, for him to answer: "If our income drops to $xxx while I can't work, and we have estimated expenses at $yyy, plus there will be expenses due to baby, how can we [together] survive this?"