Some of the replies here are really quite harsh, I'm sorry that you're getting such a hard time here OP.
I say the problem lies squarely with DH's mother, only "wanting to celebrate if it's a girl". (Does said mother love her son, despite him not being a girl?) It's not your DH's fault his mother is like this. Standing up to one's own mother can be far easier said than done.
As for DH's frustration about waiting two weeks: a wait like that can really seem like an eternity when (to him) there's a lot resting on it. Yes, two weeks is nothing compared to the pregnancy itself; perhaps it's short-term frustration, and he'll be over it in a day or two. In a situation like that, it can be terribly tempting to find some way to get control of the situation, i.e. wanting to spend £59 on a scan, just to get off the tenterhooks. Also with an impending baby, there's a lot going on in both your minds: there are many stresses now and to come.
It could be a good thing that DH is actually talking about his worries, and not bottling them up. Sometimes my DH refuses to say a word about what's troubling him, perhaps because he's afraid of what I might say, even when it's obvious that there is something on his mind. It sometimes takes a lot of coaxing to get him to say what's troubling him, and sometimes he has not said anything at all, despite all persuading, until it's much too late to do anything about it.
I'm just speculating here, but if your DH does have a difficult relationship with his mother, perhaps he is (unconsciously) also afraid of the same thing happening with his own child, which is why he feels he needs some "reassurance to bond".