If you were 42, not 22 then you might have a point, but seriously, just wait until he's genuinely ready.
Given howuch time you've got, it's far more sensible to wait until after the holiday anyway, because who wants to be on holiday with morning sickness and a high risk of miscarriages, when you could just wait a month or two.
Given your age, and lack of experience/likely qualifications (given you got pregnant with your first the time most young women are doing a levels), it's really important to get married to protect yourself. If anything happened to the relationship, you'd be extremely vulnerable otherwise.
And that's not me criticising you being a young mum - that's your choice and it's fine, but the reality is that if you split, you'd likely struggle to get a job paying more than minimum wage, and you'd likely end up surviving on benefits (nursery for 2 would cost more than your income).
Why not have a quick wedding (you could even do it before your holiday if you really wanted, and make it a honeymoon), and then come back to this is a few month/year or so. Either way, given your position, getting either married or some work experience (or both) before you have a second will put you in a much better position.