I opened this thread ready to tell you that you were not being unreasonable. But I also expected the back-story to be that you were both childless and had been living together for (and discussing having a baby together) for many years.
The circumstances make a huge difference. Two years isn't a particularly long time to be together - especially if you both already have children to consider, and the relationship hasn't yet reached the point of living together.
I honestly think it would be madness to plan having a baby with someone you haven't been living with for a while (and that's without even the existing children to consider). How do you both know you will even enjoy living together and the relationship will continue?
I also agree with everyone else that you need to consider the existing children first - and for a 7 year old in particular expecting them to adapt to so many changes so quickly isn't very fair.
I know this must be hard for you but I think your DP is actually being sensible and reasonable to want to wait. I think the harder issue - and the one I would want an answer to before moving in - is around his uncertainty. I think it's perfectly reasonable for him to say let's wait a year and then - as long as everything else is working out - let's start trying then. I think it's a different thing for him to say he doesn't know and won't make his mind up for a year. This isn't particularly about a baby per se as much as about whether you want the same things in life - and that's something you need to know before you even think about moving in with him.
So I think you need to have the conversation with him about - if everything else is going well - would he want to start trying for a baby in (for example) a year. If he says he's not sure, then I don't think you can force him but I think you need to consider - now, before you move in with him - whether you could live happily in a relationship with him if in a year his answer turned out to be no. If you know you couldn't, then I think you need to at least think about going your separate ways.
On the other hand at 38 I think you also need to think what your options are. As I said before (and as others have said) I think it is perfectly reasonable for any person to want to have some time living with a partner before trying for a baby. So even if you decided to end your relationship today, and found a new partner tomorrow you could be easily be 40/41 before a new partner felt ready to try for a baby - which may be too late. How would you feel if you left your partner and still ended up with no baby?
Would you be willing to try donor sperm insemination and have a baby as a single parent? And is something you have the funds to do?
Sorry this has turned into such a long answer - much longer than I expected it to be, but I think you have a lot to consider. But I don't think your DP is being unreasonable.