I think you need to accept that he’s not ready to get married yet and stop pressuring him.
I actually think he’s probably being quite sensible from his point of view. You’ve already split from the father of one of your children. You got pregnant very early in the relationship, you’ve bought a house (is it in both your names or just his, did he own it when he met you?) and you’re talking about adoption. He really is sensible to put the breaks on here.
You are still in the fairly new stages of a relationship where that ‘in love’ feeling still lasts. You have a young baby. There is a high probability that when the ‘in love’ feeling wears off and the reality of parenting a young family kicks in, the wheels might come off. I think from his point of view, it would probably be prudent to wait until you’ve been together for 3 or 4 years, particularly if most of the financial assets come from his side. You might not like him much when the ‘in love’ feeling goes. He might not like you much. It happens a lot.
With the rush to adopt, and him paying childcare bills etc, I don’t think you could blame a man in this position for considering that part of the rush might be to make him fully financially responsible for both your children before he has chance to breathe, so that a split would leave you in a good financial position.
The whole point of getting married before you have children is that you have both agreed pre-children that you are both financially responsible for each other and for your children, and that your property, cash and assets are shared. If you have children before you marry, you take the chance that your partner will not be prepared to make that agreement. Rushing into a baby and a wedding and an adoption, one failed relationship with a child behind you. None of these are very good signs the relationship will be a long lasting one.
I know people on here won’t approve of it. But a man in your DPs position would need to consider that if you split up in 12 months time and you’re married, is he going to be in a position of handing over a lot of his assets to a woman he had a brief relationship and an unplanned baby with, who rushed him into marriage he wasn’t sure about, and financially responsible for a child who isn’t biologically his and he only lived with for a brief window of his childhood. Or if you’re not married, he will walk away with most of his assets intact, access to his daughter, and access to your son too, if you really are serious that he views him as his father.
Much as people on here might not like it, many, many men will go for the second option. People do have the right to protect themselves from unfavourable financial fallout from failed relationships by choosing not to enter into marriage if they choose.
Probably, in a few years, if your relationship turns into a solid one, he will be much more open to the idea of marriage. If you keep on trying to railroad him into it, you may well find that he finds walking away a preferable arrangement to being railroaded into a marriage and adoption he isn’t ready for. Especially when they offer lots of benefit to you, but only risk to him.