I'm going to quote your OP, so there is no doubt in your mind that I have read it:
"DP and I are in our twenties, and I am 13+5 pregnant, due in November. His friends have decided to book a weeks holiday end of August, start of Sept... DP asked if I would mind him going and I said yes as il be 7months gone I have DS to previous relationship so will need the help and money etc..."
Firstly, congratulations on your pregnancy. For you, though, this is second time around and you know what to expect. You are a committed (because you have to be!) parent. However, your DP isn't really. Although you say later on in the thread that he has taken responsibility for your DS (which is great, surely?), it isn't quite the same for him as having his own child. He also hasn't (I presume) lived through a pregnancy and will have very little idea of what it is or could be like. And even if he's read the pregnancy books, reading it in black and white is very different from experiencing the whole thing.
If you have been fit and healthy so far in your pregnancy - and we don't know because you haven't said - then he will be assuming that you will be fit and healthy by the time this holiday comes around. I would imagine that he also thinks that there is plenty of time from the end of his holiday until your baby is born.
I'm wondering why you think you will need him to be around so much to help you by the time this holiday is coming? Are you likely to be poorly in some way? Did you have issues in your previous pregnancy? I'm guessing not, as you haven't said you did. In which case I have to ask you why you want him to be around to help you? Surely you can cope with your DS (whose age we don't know) on your own for a week?
Re the money, you have gone on to say that you have separate finances, and that you earn more than he does. So is he spending 'his' money on this holiday? Do you have any joint finances? This to me sounds more an important issue for both of you to talk about. Presumably you will be taking a fair amount of time off for maternity leave, yet you are the main or higher earner? What plans have you both put in place for money to cover you over this period? Have you talked about it?
If you have discussed it, and there is sufficient money to cover your maternity leave, and allow him to spend his own money on whatever he wants to, then what's the problem with him going on holiday?
If you have discussed it, and there isn't sufficient money, then he is being very unreasonable to want to spend that amount of money on a holiday just for him.
If you haven't discussed it, then you are both being unreasonable.
"So me being me said he should go for a long weekend and come home before his friends, stay 3nights, that way we are both happy!"
Well, no. You won't both be happy. He will still have spent a considerable amount of money, so you won't be happy. He will be forced to leave his mates behind half way through the holiday, or will only be able to join them half way through, thus missing some of it, so he won't be happy. This really isn't a good compromise.
"His mother advised us "he should go as it will be his last" and he keeps repeating that now complaining after agreeing to the long weekend he wants to go for the week! "
I do have some sympathy for this point of view. Life will change for him, inevitably, when his child comes along. It also reflects the fact that this is a big test of your DP in terms of how he chooses to behave when he becomes a parent. Is he going to grow up and take full parental responsibility, or is he going to leave that to you and continue to behave like a single bloke?
Have you talked about this? Was your baby planned? Do you have any idea how he's likely to be with the baby and the responsibilities that will come his way? The fact that you seem happy with how he is with your DS implies that he should be a good parent and will 'step up to the plate' with his own child.
And you say that he did originally agree to the compromise, which suggests that he did understand your POV. Sounds like the issue you have here is more with his mother!
"First of all I will be 7months, I have DS I need help with and the money would benefit the baby, flights and accommodation are costing £300 and spending money, well, at least 2weeks wages! We are far from flush!"
If you are fit and healthy, and have had a previously fit and healthy pregnancy then there is no reason why you won't this time around. So this isn't really a reason to stop him. You haven't said where he's going, but if it's only for a week, presumably it's not that far away that he couldn't get back quickly. Have you looked at any contingency planning like flight times back, etc?
Why do you need help with your DS? If you can't cope with him now, how are you going to manage when DC2 comes along?
Obviously the money would benefit the baby, but what financial plans have you already made (as per points made earlier in this epic post)? When you say you are far from flush, how is he planning on paying for his holiday? Can he afford it or would he be using credit cards to pay for it? If the latter, then he is definitely being unreasonable, as well as foolish.
If he can afford it, and it's just depleting your funds somewhat, then you are being unreasonable.
Overall, it's impossible to say if YABU or YANBU because there are too many issues that you haven't actually disclosed here. Therefore, the only replies people can give you will be based on their own views and experiences. And, as you have seen, these vary enormously. Yet everyone who gives a view different from the one that you want - that YABU and your DP is BU, are rejected by you.
I, personally, would let him go - but only if you have sorted your finances out and know how you are going to manage the period of your maternity leave.