You need copies of his P60's for the years they are claiming back, then put the amounts into the calculator that someone posted further up so you know a ballpark figure.
If he is on just over £50k it won't be a huge amount - I think one year we had to pay back less than £100 but now DH is near the £60k mark, we now have to pay back nearly all of it.
Depending on when you speak to them it depends if they agree a payment plan or not, they would not accept one from us as it was near year end, but we were able, just, to pay it all back by an agreed date.
Make sure you do any tax returns before 31st January as that is when fines etc kick in.
You may also get some late fees or fines so the sooner you sort it the better - I don't think we had any fines but did have to pay some interest, I think.
Below £60k you still get it, but you will need to pay back part so you will have to decide after you get the figures together if you carry on claiming as you can still get the pension credit part you just opt out of receiving the money.
If you go onto gov.uk/child-benefit-tax-charge it will explain it all.
This is one thing the government need to sort out as the thresholds are so unfair and out of date.